Man, how does a young guy like you get a job like that? Sounds like a good, challenging design job.
As I promised, here are more detailed instructions on what you need to do to get your kit built:
Tailpipe Welding Instructions
Make your mounting plank and heat shield as shown on the drawings. Mount the front end assembly on the plank using four short bolts and tee nuts, with the heat shield sandwiched in between. Make sure that the two small holes at the rear end of the shield line up with the small nail holes in the plank!
Cut your tailpipe from the antenna mast material. The easiest way I have found for accurate 'square' cutting of tubing is to wrap the straight edge of a piece of heavy paper clear around, then trace it with a fine-point permanent marker. SLIGHTLY flare one end of the tailpipe tube, so it will just fit over the rear of the front end assembly. Tweak it a little out-of-round to fit over the two weld bulges visible in the rear-end photo posted earlier. It should have just enough flare to center itself on the assembly, maybe a mm or two! A close fit is important - take your time to get it right!
Now, slight the SS wire strut onto the other, un-flared end of the tube. This should be a slightly snug fit, and it should go on easily. Slide it on a couple of inches without distorting it.
Place the flared front end of the tailpipe against the mounted front end assembly so it fits together properly, and gently position the two legs of the wires strut against the metal heat shield so the rear of the tailpipe is supported. Temporarily tape the pipe to the front end assembly with a 4-inch length of masking tape.
Gently slide the SS wire strut back or forward to line up its two loops with the screw holes. With a small screwdriver, run in a no. 6 panhead sheet metal screw, with a no. 6 flat washer, into each of the two holes. Run each screw down tight to hold the wire loop, but do not overtighten.
Set up the plank and mounted parts on a level surface for tack welding. If your mounting plank is wider than the heat shield, protect the exposed surface from weld splatter with scraps of sheet metal, aluminum foil, or other non-flammable material. Use a heavy object, such as a brick, as a stop against the front edge of the plank. Use a similar object as a stop against the rear end of the tailpipe, pinning it against the front end assembly. Once these are in place, remove the tape and make sure everything stays securely in place. Proper orientation of the pipe against the rear of the front end assembly is the critical point.
This is the point at which you can form a shield of several layers of aluminum foil around the intake flare to protect it, if desired.
Make a small tack weld at the very top of the joint. You should use very little filler rod, instead trying more to blend the flare down into the rear edge of the front end assembly. Duplicate this with a small tack weld at the leftmost point, and again at the rightmost point. These should be fairly small but solid, well-formed tack welds.
Once you're sure this is properly done, remove the front and rear stop blocks and remove the two small screws from the SS wire rear strut. Then, remove the four main mounting bolts, and carefully remove the engine from the mounting plank. Set the mounting plank and heat shield aside, out of the immediate work area. Turn the engine over on its back and make a fourth tack weld, right at the bottom of the flare.
Imagine the four zones of the joint, between the tack welds, as compass directions NE, SE, SW and NW, as seen from the rear. Starting at the bottom center tack weld, weld the SW segment by melting the flare into the adjoining metal of the front end assembly, using as little filler rod as possible. This should be smooth, continuous welding until you have blended it into the side tack weld. Now, turn the engine over (180 degree rotation) and weld the NE segment in the same way, from the top tack weld through the side tack weld. Now, turn the engine over so the SE segment is visible, and work from the bottom up to the side, blending it into the NE weld just completed. Finally, rotate again and finsh the NW segment. Working the welds in this diametrically opposed manner will preserve the initial alignment almost perfectly. You should end up with the appearance of one continuous weld clear around the joint, without holes or gaps of any size.
Remove the intake shield, if used. You can use a small flat or half-round file to clean up any sharp points or other superficial problems on this weld. That's all there is to the tailpipe weld.
Starting Air / Fuel Tube Welding Instructions
Again mount the engine and heat shield on the mounting plank. Run in the four main mounting bolts first, then the two small screws at the rear strut.
Slide the long straight end of the starting air tube through the mounting rings from the rear to the front. Rotate the tube up (clockwise as seen from the rear) so the fuel pipe enters the intake flare as you continue to move it forward. Note that the long, free end of the fuel pipe is rotated up away from the area of the mounting rings - this is to protect it during welding. Stop the forward motion when the nozzle end of the air tube is 1/2 to 5/8 inch from the intake flare. This is the correct position, longitudinally (front-to-rear). Raise and lower the nozzle end of the air tube until it you can see where it is exactly centered between the two sides of the intake flare. It should appear closer to the bottom than the top of the intake opening at this position. This is the position you MUST have it in when you weld.
Use a small C-clamp gently clamped onto the tailpipe as a rest to establish this exact position. Take your time in working this out! Remember, what you want is the air tube precisely centered from side-to-side in the intake, and 1/2 to 5/8 inch aft of the flare edge. Once you've established the right place for the C-clamp to be, tighten it slightly on the pipe. You can rely on gravity and friction to keep the pipe resting properly against the clamp, but I'd suggest wrapping with fine wire to hold it in place (your welding will be about three inches away from the intake area, so thin copper or aluminum wire should be OK to use).
There are actually four weld points to hold the air tube, and they are easy to reach. Your main difficulty will be that the air tube is VERY thin steel - only about half the thickness of the mounting rings it fits through. So, at each weld point, you must concentrate your heat on the mounting ring, add a little filler rod, and "pull" the weld down onto the tube and quickly blend it in. (It sounds more difficult than it is.) The four weld points should be obvious: Each mount ring has a lug bent down over the tube as it goes through; the end of this lug is one weld point, and the top of the hole where the lug is bent forward is the other. (This will be MUCH clearer when you see it in real life - trust me!) The welds need not be extensive - something like large tack welds is all we're after.
Re-check the perfect side-to-side alignment and longitudinal spacing of the air nozzle at the intake. Once you're sure it's right, go ahead and make your first weld at the rearmost weld point. Work your way forward, doing each little weld in turn. When done, remove your wire wrapping and C-clamp from the air tube and tailpipe, respectively.
At this point, the long straight section of the air tube lies a little close to the front edge of the combustion chamber. You may gently bend it out a little, if desired. Now is also the time to gently bring the long, free end of the fuel pipe down parallel to the long part of the air tube. You can put a gentle outward bend in the last couple of inches of it, also, to keep your fuel hose away from the hot chamber wall.
All that's left is to align the fuel pipe inside the intake. What you do is use a small flashlight, and sight down the intake from every position you can get your eye in, and judge where the VERY END of the pipe is, in relation to the upper (curved) wall and bottom (flat) wall of the intake. Like the starting air nozzle, it should be exactly centered laterally and a little lower than center vertically in the intake cross-section. GENTLY use the grooved end of the special tool to leverage it one way or another to get it properly centered. It is VERY important that the end of the pipe be well-centered in the area of the intake, for good starting and wide throttleable range, so be ready to take your time and exercise patience with this step!!!
That's it. You should now have an engine that will start and run. The only additional step would be to provide a tail-end flare - but I would hold off on that until we're sure that the length is just right.