[87BOSS351C]

Don't waste your time with a 351M (the M doesn't stand for anything). Any cleveland head will fit it and some other c parts. You can put on the 4V C heads but you have to run a 4V C intake and spacers (the M and 400 have a more than 1" taller deck height than the C) and you will have to have custom made headers. The cam and timing chain and gears and cover and oil pan will all interchange between the M and C. Rods, pistons, crank and push rods are all different. Also, your bronco (78) will have the crappy smog 2V heads. These are basically the same as a 2V C head but it has a big bump in the exhaust port. Also the flywheel and clutch from the stang won't work (the 351M uses 28.x oz balance). And the M motor mounts are 351M and 400 only bastards. The bellhousing pattern is the same as the 429/460 so your tranny won't even bolt up.

The 351M/400 is great for a 4x4 cause it has good low end torque and not much else. They are part of the 335 series engine family which started with the 351C in '70 (400 in '71, 351M in '75 to replace the 351C which ended in '74). The 429/460's are the 385 series. Really Ford never called them Big Blocks and Small Blocks back at that time. The 335 series and 385 series will interchange distributers and rocker arms.

You would be looking at way more work than gain with that swap.