73' 340/904 duster racestoration

Just cruisin around and saw your post. By the sounds of things you're on the right track. Sounds as though the 360 has ample power if it's spinning the wheels in first gear as you first stated. The rear swap will help as you have noted but the body is still going to retain some un needed flex do to torque. Frame connectors will pleasantly suprise you if you should get an opportunity to put a set in.
Small Block