Mopar Small Block, 904 Torqueflite Transmission and an A-Body (65-72) 8.75" Rear are the easiest to fit in. A Body 8.75's are hard to find though, you can use a B Body 8.75 though, but it's a little wider than the A or stock D50 rear by 1.5" on each side (great if you wanna push the wheels and tires away from the frame to fit fat tires under her)

I've got a Mopar 273 / 727 in mine