[THEFERMANATOR]

Looking at that picture tells me you MOST likely will need a complete transom in that boat. You can see the silicone gooped up on some old transducer holes, and there appears to be some cracks over on the port side where the aluminum is missing. I' malmost willing to bet that boat was pulled out of service for a reason looking at that picture. And no, your 20" shaft won't directly swap over. You would either have to convert yours to a 25", or cut that one down like your 83 is. First thing you need to do is get that aluminum off and look in there and see what you find. But make sure and keep it covered over, many a transom has gone bad because people left that cover off.