The 351 fits in the bay just fine, it is a bit tight, the only thing you would need would be some swap headers. Manifolds will be a pain to get off when you want to change to headers down the road, so if you plan on going headers at some point do it now. The bolts on the manifolds are long and the longer one will contact the shock tower when you try to take it off. I had this problem in a 69 and had to chisel the head off the bolt, so I imagine it would be a lot worse in a early stang.