Help me win this argument 400 in a 72.

With all the jack you're gonna spend on the 400 and trans purchase(s), you could yank the 318 and build it into a little fire breathin nasty beeotch and use the same transmission, motor mounts and accessories. A 325HP 318 will move the hell out of an A body and also keep you in the family's good graces all the while being the most gentle on your wallet.