2v heads vs. 4v heads?

[thunderbolt]

My understanding is that yes the heads would interchange but you would then have problems with manifolds since the 351M was wider and the 4v heads have much larger intake ports, the 351M mainifold would have terrible port miss-match. I guess my question is why? The 351M is not exactly regarded as a good foundation for a proformance engine, where the only limitations to 351C's are minor oiling probems and thin wall casting (all of which effect the 351M if I am not mistaken).