[Capri306]

Save yourself the heartache and money, and go with a 351W-based engine. Whoever told you a 351W is a bad choice for building upon must have been on methamphetamines. It's probably the single-best block to start a project on, and interchangability is nearly a non-issue when fitting them into Mustangs. I won't lie to you, there are some minor differences that need to be accounted for, but most of the time it's just a matter of selecting the correct parts and not performing extensive modifications.

A factory 351W block of 69-73 vintage is usually capable of handing around 700-800hp (some say more, but I like to low-ball) before needing some help to stay together. I have one of these blocks right now, and it's...shall we say, "beefier" than ANY 302 block I've encountered.