Had the same problem in the past with a '76 Gran Torino I had with a 351M. I wanted to swap it out for a 351W or something along those lines, but I would've had to swap out the entire drivetrain, and I just didn't feel like farting around with it back then. *kicks self in arse for not keeping car*
FWIW, my Torino had a C6 behind the 351M, as well, and I'm pretty sure that was the norm all across the board with other Ford cars and trucks during those years.