the big thing about an FMX as you can see is the weight. And probably uses 20-40hp over a C4, (just a SWAG).
I ran one behind a cleveland for a number of years in a pinto body. we ran 7.40's on a 1/8 mile tracks. never had a moments problem from it. 351c was probably makin 350-425hp or more with 5.67 gears and 12" slicks.
the fmx tranny is strong as hell. I knew a guy with a 400 horse windsor that would throw the fmx into first going 60mph and smoke the tires down the freeway. However they are not very common and unpopular due to their weight. I would just go the built c4 route.
when i had the one in my torino rebuilt, the guy at the tranny shop said the fmx was bad about grenading the planetaries, don't know if he was full of **** or not as mine were ok. I do know that there isn't much available for hop up parts for the. try trans-go, they make shift kits for them