for whatever reason, those 400f bikes are fetching a #$%*ing mint in japan in the last year. And by that I mean $6k+ for sort of okay bikes, $9k for a mint survivor. If you can find someone who knows japanese, and can write you a decent classified, you might solve some of your problems.
In the meantime, putting a 400F in a rebel frame is a fools errand (especially after swapping your welder). Even if you make it fit, it'll torque that thing into an oscillating pretzel in no time flat, and be a dangerous, fast road hazard.
A rebel is like a bic lighter..