what year wagon rear ends are interchangable

i have a 90 old custom cruiser wagon, what year and model rear end will fit my vehicle? 8.5" posi? discs? what gears came stock in a wagon with a posi?

Different gears came with the car, but as a general rule, '77-'90 should work. '91-'96 MIGHT also work, not sure.

Most of the ones I've seen are 8.5" but don't be surprised to see a 7.5" in one.