An adapter could be made, but I believe that it is more work than what it is worth. For starters, the G58 was only offered from 1989-95 in trucks that had Autolocking front hubs for 4WD shift-on-the-fly ability, which limits the amount of customers we see who need parts for a G58. But more detrimental to this problem, is that the transfer case shift rail height is lower for the G58-chain drive t/case setup than with the RF1A gear drive t/case setup, so even if we did make an adapter to put a gear drive t/case behind a G58 transmission, we would either need to make customer shift rails to account for this height offset, or we would need to make a special shifter that had a longer throw (which no one would want).

So in general, we strongly recommend you to consider upgrading (a W56 is stronger than a G58) to any W56-type transmission from 1985-99, understanding that there are two basic models available, one from 1985-88, and the other from 1989-95, short and long forward shift t/case rails respectively.