couple of things... 429/460 didn't come in F150 or F250 4x4s from the factory. it made the front to heavy. the majority of BB stuff came as trailer tow packages in the Camper Specials F250/F350 stuff. even the F600/800 stuff have the luxury of the 377/429 package because they rev high for the high toqure at low speed.
If you had a Cougar they would accept the 460 untill they dropped the body size in `80. theres some very rare 460 bellhousing on C4's if you find one the swap is notheing more then engine mounts and brackets. the C6 is about 8" longer and with all the high performance stuff you can build the C4 to ba as strong as a C6. I did the BB swap in my buddys cougar and we had to cut down the driveshaft to get the C6 in.
Muddypaws. If I remember right the 351C ended its production run in `73. the 351/400Ms started in after that. so if you have one after `74 its most likely a "M" engine.