66dart gt needs new motor

As said already, a 318 360 will fit in there. I put a 318 in my 66 Dart 270 plus put a 4 speed in it too. was a auto. Had to weld in the hump for the 4 speed, but wasn't really all that hard. And yes, you would need to upgrade your rear end for that kind oh hp. As for the drive line I know you would need a new one if you go to a 8 1/4 or 8 3/4 rear. Let us know more about oyur car, and post some pics of it for us to enjoy.