Say I had to settle with a 302

[thunderbolt]

Yes you can for all intents and purposes put a 351 in an early model mustang. A 351W will in most cases bolt right up. If you are aiming for 400hp regardless of engine cubic inches, your whole drive train will need to be gone through. Some other thoughts. Up grade brakes. You have made it go fast, now make it be able to stop all that fast. Don't negelect the body strengthing. Sub frame connectors, export brace, monte-carlo bar, maybe even a roll cage.