Help me win this argument 400 in a 72.

Ok, so the wife has a 72 dart with a dying 318. I found a running 400 with 2 trannys for cheap. My father in law says pass....

So what all is needed to shoe horn this puppy in? Headers and motor mounts? Will the tranny bolt in just fine?

Basically I need you guys to make a quality post so I can show my father in law? And can you guys say what's needed or has to be done? Yes I know how to search but my father in law doesn't want to read 8 different threads....

And I'll be using his rig to haul it, his shop to install it do yea, he's part of tge equation...

I see "wife's Dart" and her dad doesn't want to install a 400 in his shop. Do you need to be hit with a bigger stick?

Bolt in a small block.