Somewhat limited with the short wheel base of the 80. Most likely you have a Dana 18 transfer case, with texas style mounting bolt pattern to mate to trans. Unless it has been swapped out previously to a different t.c. One option, something that I'm not sure if IHPA sells, is warn overdrive for the Dana 18 transfer case. Call them to find out, cause I didn't see it in the online store. I know a few people who have it and seems to work well.
More expensive alternative, if ya got deep pockets, is do a overdrive trans swap. But then ya need to consider adapters to mount to engine and transfer case. IHPA does sell the adapters, depending on your needs. Then consider new prop shafts and driveline angles and new transmission cross member mounts. All feasible, but pricey. May take a decade to recoup the savings in fuel cost