Best engine for my 1972 Mustang?
The bigger and heavier the engine? The harder a 72 Mustang will handle. You'll need to beef up suspension. Get no more power than YOU can handle. That car will toss you into a telephone pole. Or, sailing off the road in a corner faster than you can blink. Untrained. So, don't just toss a 500hp motor and think both you and it are now All That. You won't be. Even though you make the car a beast. Get experienced with it on a track before you go street warrior with it? I mean that nicely too. Seen too many fine cars with with green drivers get into a mess.
Anyway, The finest stock Mustang setups involved the 427, 428 and 429's You may increase horsepower at will with these. Modern setups seem to like the LS-1 series. A great balance is a strengthened 302/351 series with EFI and high compression.
Select your flavor engine/trans setup and have fun!