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I'm not sure if the c6 was longer or not. Might have to move the crossmember and shorten the drive shaft. I don't know for sure. It's definitely not a hard swap. They were factory equipment.
Personal opinion but, a C-6 would be a better choice.
X2 I blew 3 of them in two and a half years behind a 351w in a 69 Mustang before I swapped in a Toploader. Of course, I was only 18 when I bought the Mustang and drove it like a teenager would.
I have an FMX in my truck, the driveline (351c & FMX) came out of a 73 Mercury. Driving down the road yesterday going about 45mph I accidentally hit the shifter while leaning down towards the floorboard and knocked the tranny into reverse. The tires screeched the motor died I jerked it into N as soon as I realized what I had done and let it coast for a second. Restarted the motor and put it into drive and it's still working fine. I never thought much of the FMX but now I have gained some respect for it.