The FMX trans was a real pos in regards to performance transmissions. They made millions of them for many applications back in the day but they were very heavy compared to a c 4 trans so most performance transmissions were built c4s for small block cars. Ford made a small block c6 that came out I believe in 70 or 71, a great transmission back in the day that had many upgrades that could be had, like custom valve bodies, shift programs, converters etc. Then there was the BEEFY c6 for big block applications which is a tuff hombre but very heavy. Most racers use a modified c4 that can easily handle over 1000 HP these days. Times have sure changed.