OK, now you've don it
First of all, who said only 250hp? My wife's 70 Mach 1 with a 11:1 compression 351C-4V came with an FMX, and that has (supposedly) 300hp from the factory. By the way, it isn't horsepower that you have to worry about. It is the torque handling capability.
Also, I have a buddy that is still using the same FMX tranny that came in his car originally. His car is a 71 Mach 1 that came with a 351C-2V. OK fine. Afrter going through several iterations of performance engine rebuilds to the Cleveland, where he got down to the high, high 12s in the quarter mile, he put in a 514" stroker motor. Still the same original FMX! Why? because 'they' said it couldn't or shouldn't be done. He used a bellhousing from a 351M full size wagon I had once. There have been several modifications to the trans, like a much higher stall converter, shift kit, etc, but it is still not all that far from what any weekend mechanic would do. This car is now turning 10.90s (actually a little better) in the quarter. I'd have to guess that it is putting about 500hp to the ground, easily!
The FMX has a cast iron case, so that is plenty strong, but the big plus is that it has a center support bearing for the internals. This bearing keeps the internals from flexing or bending under extreme duty. Like I said, they're just about bulletproof! Just about any of the performance aftermarket companies still have the rebuild kits, shift kits and performance converters available, they just don't advertize them any more since the FMX is pretty much gone for all intents and purposes. The one thing you cannot get for an FMX is a trans brake. I believe the last usage for the FMX was in the late 70s.
The point is, don't give up on an FMX.