Nv3500 vs ax15

My floor is rotten and I want a 5 speed. I know neither is gonna fit into my 64 dart but I'm ready to cut the tunnel out and make it fit. It's going a behind a 360 magnum but this is just a very stock street car so I'm sure I'll be fine. These two trannys are cheap and everywhere in old dakotas. Any ideas on which one to go with. How substantial is the size difference?