I bought a 94 pushrod 5.0 V8 convertible with an automatic transmission. The car is in pretty good shape, but the transmission will not shift past second gear. In the event that I can’t find the problem, or the problem is internal and necessitates a rebuild... when I replace it with the abundant 4R70W found behind the later 3.8 V6 mustangs what problems if any will I run into?