The problem with using an adapter is that I would have to mover either the engine mounts forward or the Xfer case mounts backwards, along with changing the lengths of both driveshafts and moving all of the shift linkages.
More than likely with any tranny swap you will have to relocate and / or replace drive shafts, the 3 speed is a short box, probably has a T-18 transfercase. You will have to adapt that, and if upgrade to a D20 or a D300 you still have the same issues.
That being said, look at the Ford T-18/19 series of transmission.
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