Not sure about what Bones is talkin about, but.....
On my 80, a PO had swapped in an Lseries 5 spd, and all I had to do to swap in an R151 was to pull the old tranny, and put the replacement in. I think G52, W56 and R151 are all the same length. The r151 was just under 5" longer than the L-series. Youll have to relocate your stock crossmember or fab one up, or buy one from Budbuilt. You'll also have to cut and lengthen D-shafts.
Other than that, should be a basic pop & swap. You might be able to use your old clutch if its in good shape, too, but since its a 4spd, dont hold me to that. I know if it was a 5spd you can.
you might have to enlarge the shifter hole in the sheetmetal, if the 4spd shifters are not in the same spot as a 5spd.