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Is you 4 speed floor mounted? If so it is the granny 4 speed used in both V8 and I6.If you want to upgrade swapping heads is fine, BUT you will need to have all the new parts ready as much of nothing will swap from the integrated head. That being said, there is nothing wrong driver wise with the integrated head.
Sounds like the Overdrive trans. Does the shifter have the following pattern.R 1 3 2 OD
Those were used by GM from 81-87 behind V8 and 6 cylinders both. As long as you aren't beating on it, they will work fine. Generally they are prone to leaks due to design. There is a post on here with someone rebuilding them. IT is a NP A833 transmission.