any input

You explained it very well when you stated you didn't know what your doing so i'll be as gentle as i can. First off your rear diff is more than likely a 7 1/4 rear. Replace that with an 8 1/4 or an 8 3/4 unit and you'll be set. The 5 speed that you refer to never came in a 75 A body. It may be in it but it never came with one. The car you have is an A body and as your fender tag denotes came with a /6. The pedals you have in your car will work with an 833 4 speed unit but you will have to switch the shifting to the floor. As far as motor , that will depend on what kind of driving you will be doing. 1/4 mile? Cruiser? Touring? Tell us more. Hows the rust? Where are you located. There may be some members here that live close that can share some knowledge before someone skins you alive. If you don't know ask questions here and someone will respond. Be careful and good luck.