1fast, that looks really good. Yes, that WILL be a ramjet - IF you can move enough air into it. The diffuser cone is really nice, but that area ratio means you'll need some really fast air to get full power out of it. You have a fairly high speed design there, because the exhaust area is so much larger than the intake area. That means you will need to get pretty fast air going in to achieve real pressure in the chamber, and thence, good usable thrust. Adequately testing this will be the tough part.
The problem always seems to be that it's not easy to get that large a cross-section of very fast air, clear across the intake. If you use too narrow a stream, so there's some edge turbulence inside the engine's intake rim, you won't get good diffuser action - you MUST have an airstream somewhat larger across than the front rim of the diffuser cone.
It would help to put a SLIGHT intake flare on the front rim - you don't want too much, because whatever you provide will reduce the effective area ratio of the diffuser.
I like this one very much. Intuitively, the flameholder looks about right to me.