auto trans C6 vs. FMX vs. C4

[Kcode351]

I pulled a 351w and auto trans from a 76 Cougar. I always thought it was a C4. I had the torque converter reworked to get some stall, but never ran it. Recently, I gave the trans to a friend. He said, the shop told him it was a FX, and would be difficult to mate with a late 5.0. I'm assuming he meant a FMX. Now, its back on the floor of my garage. After a year, the 351w now has several mods. Could some one give me comments on C6 vs FMX for performance and durability on a weekend driver.