Will a 77 Charger 360 fit in a 65 Valiant Slant Six Painlessly?

Conversion motor mounts, correct exhaust manifolds for the A body, or make Magnum manifolds work, and you will need the transmission from the charger. Larger radiator. A few wiring changes. Upgrade the differential and when you do, you'll need a shortened driveshaft.

That's the basics.