You could also get a 400 with a small block bellhousing, but only in 74, and it was in front of a FMX.
351M's really aren't worth much more than scrap value, simply becuase they are EVERYWHERE. Now 400's, quite a bit harder to come by, and really have some potental. It's hard to go wrong with a 4" stroke.
The big issue with them though, is the piston is nearly .070" down in the hole, so if you just put flat tops in them, they have detonation issues. The key is to either zero deck the block, or to get the pistons with a taller compression height to fix the problem. the two thing's a M motor lacks from the factory is compression, and cam(mainly cam timing)
My buddy put one together for his rig last spring that runs really, really strong. 79' 400, stock compression, .519/,522 lift cam, performer 400 intake, ~780ish holley, and headers. Runs very strong, and builds gobs of torque.
Seen one at the mud drags last weekend that made really good power as well, 10.5:1 compression, aussie heads, and some other goodies.. very fast bronco..