Yeah, the M's were strong motors, and you can get buttloads of torque out of them. The black bronco in my pics and vids is powered by a 400M. The only down fall of the M motors is not as much variety in aftermarket parts.
As for my 460, it reciently was rebuilt. .030 over with sealedpower pistons and rings. The head deck on the block was evened up, meaning, even from the factory, the deck height might vary .010" from front to back. It helps keep all the cylinders at the same compression.
My motor came out of a '76 lincon, so it's got the D3VE heads. Look for those, or the D0VE heads. I had them ported out.
Cam is mild, for off idle power, intake is a Weiand stealth high rise
600 cfm edelbrock carb. No need to put too big of a carb on, this one has never been not enough fuel, even when at full throttle in deep mud for an extended period of time. (hehehe that brings back warm memories)
OE style dist, cap and rotor, msd wires, accell blaster coil, autolite copper plugs.
Guy's at the machine shop, (Knowlton's Thunderheads in bethel, MN) said that this will produce 400+ hp, and 500FtLbs.
I believe them, this thing has ALOT of grunt, even with high gears. I was thinking I would change out gears to 4.56's, but I havn't been in a situation to need them yet.
I was through 2+ feet of fudge-like mud this fall, and was able to spin all four tires as fast as I wanted. Only got stopped cause my bumper hit dry ground while going through a hole at the far end where everybody was getting stuck.