The 64 Galaxie also came stock with a 390cu V8 C6 automatic, and they are easy to find and bolt right in, may have to change crossmember for tranny,,and of course drive shaft.
But realy,,,you could go all the way to a 460 if you want.
Disc brakes, and spindles can be taken from a 76 cougar,,,you will have to change from your stock wheels though, cos they wont fit over the Disc brake calipers,,,,you will also need to upgrade to a dual circuit master cylinder from a newer ford.