the early block is different so the later engine wont bolt up. the lowest bolts are in a different spot. alsothe early flywheels have a different tooth count so you have to have th starter to match. I had a bad bendix onthe 61 and repalced it and that one sounds bad but worked. I figured ti was jsut a cheap mexican part but when Marty bought the truck he had more issues nad pulled it and it had fried the ring gear! maybe you cna get away with an inframe rebuild for now? pull heads have them rebuilt. pull pistons hone and ring and replace all bearings. would problaby last another lifetime.