4 speed, 8 3/4, gearing questions

toolmanmike , nailed it.
remove the carrier from the truck rearend (there are 3 styles of carrier cases; in order of beefyness #741, #742 & #489)
find an a-body 8.75 rear housing & axles and your in business, less the truck gear set is trashed..post install/driving down the road would not be the ideal time to find that out.
Having said that, the 3:23s with the low 1st gear 4 speed should make for a very good set up.