Well, you've been getting some good advice here, but I don't think any of it solves the immediate problem (for which you can basically thank me). I think the idea for this kit was flawed, and I think we've basically proven that my critics were right earlier on: It doesn't make sense to provide welded lightweight pulsejet kits in a "beginner's" market. Even if the kits were made much more demanding and I TRIED to market only to experienced metalworkers, tyros would still buy them, and I would spend all my time as a one-man Help Desk trying to get everything to come out right. But, back to your immediate problem.
One thing I think you ought to do is get a tailpipe made up out of thinner tubing. That will require skilled welding, but it's not as bad as welding thick to thin material, which is a quite advanced skill. I would be happy to supply a tailpipe out of the material I use (i.e. just as if I had made it part of the original kit), or you can shop for it (try Menards this time). This problem is actually my fault, because I assumed that since you can get antenna mast tubing anywhere, it would be the same everywhere. Obviously, this was a silly assumption, and I'm sorry for that. I should have simply made a pre-cut tailpipe part of the kit. Duh ...
One other thing I thought of that might help. The thin air tube going through the rings is a "thick to thin" area that we can't do anything about. However, it doesn't need to be welded - it could be brazed or even silver soldered, because it's in an area that will never get red hot as the engine runs! This requires torch work, of course, and use of a high temperature flux.
Ask yourself this question: "At this point in time, what do I REALLY WANT?" That's the key question. There are three viable answers I can think of: You want an engine that runs SOON, without the added expense of buying equipment and learning delicate welding skills; you want an engine that runs SOON, after spending some time and money getting professional help to finish assembling it; OR, you want an engine that runs SOMEDAY, after you get the equipment and training to finish the job yourself. These are all perfectly reasonable desires for a successful outcome.
You should not feel ashamed at this point if you just want the engine finished up so you'd have something to experiment with. All you'd have to do is re-package the front end assembly and starting air / fuel pipe assembly back in the original box and send it to me via Priority Mail. I would complete the assembly (giving this TOP priority) at no charge and mail the finished engine back to you, and you'd have it. All you'd be out is some shipping charges. I think we've proven everything we're going to prove about the "beginner's kit" aspect of the project, and there still might be some important things to learn in getting it to run. So, I have no qualms about just finishing it up for you (would take me about half an hour of welding, plus a little setup).
And, there's nothing wrong with letting a pro handle the finishing up, as long as you can find someone you can trust to have the skills to do it. But, be prepared to shell out some bucks for the quality of work you need. Again, I'd be happy to provide a pre-cut tailpipe at no extra cost, which has got to be better than using a heavy walled pipe! But, if you really want to dig into some training (maybe Adult Education classes or some such - those are really good value if you get the right instructor), then you'll have to figure out how to get hold of the equipment you'll need once you feel skilled enough to be ready for some thin-wall practice. If that's your choice, I'll not only provide a tailpipe but a few scrap pieces of the thin-wall tubing you can use for practice. Or, as I said, you can shop around for "the right stuff" in your area.
Note that whatever you decide to do, I'm still holding you to a written evaluation of the kit concept on the Valveless thread, even if the final opinion is not exactly super favorable!
Anyway, think it over and let me know what you want to do. I just wanted you to know that I'm still willing to help you try to get the job done, and not just leave you hanging with an unfinished project of no value. I do consider the kit concept validation phase to be about over, though - except for your written evaluation.