Help me win this argument 400 in a 72.

I would stick with a small block in a situation like this. The car already has a small block so swapping to a 360 is a simple bolt in. How fast do you want/need your wife's hot rod to be? Even a 318 with some well thought out parts will run pretty good. I'm not sure if a 72 318 Dart came with an 8.75 axle or the puny/weak 7 and whatever axle from the factory but if not an 8.75 you will need to upgrade to something better anyway. Why not deal with the axle first. Look for an 8.75 or even an 8.25 out of an A body and get it in there with a posi and some gears like 3.55s or 3.91s. After that all you need is a decent low buck small block, say even a Magnum 318 or better yet a Magnum 360 with a few simple bolt ons and an 11 inch converter and a nice tire burning 13 second car is easy. I'll bet your wife and father in law would be more happy with that than a big fat can of worms that cost alot more.