Why? It would be so much easier to buy a Mustang II with the 302 already in it - many more of those are still around than the ones with the 4 & V6s, anyway.
You will end up putting way to much time & money into this project than it is worth - believe me. Been there, done that to a '76 Mustang II, swapped the V6 for the V8. It was a major headache, then I gave up on all the hassles and bought a 1979 Mustang
Ghia with the 302 already in it. Good choice!
Drive the 'Stang they way it is & save up your $$$ for it's repacement.