I know the answer is probably here in the thousands of pages but

First figure out which bolt pattern you want (I'd go 5x4.5, much much bigger wheel selection). If you go with the BBP, just swap out and junk the 7.25 rear, and get either the 8.25 (it will have BBP already) or the 8.75 rear (you'll need to either order new axles or if you get a B-Body 8.75 from 68-70 you just need to move the spring mounts in 1/2" each side). That said just look under the car and confirm which rear you already have before buy anything because someone might changed it already. One advantage that the 8.25 has over the 8.75 rear is if you go hit the junkyard and hunt down a 96 and older jeep grand Cherokee you can pull the suregrip (limited slip diff), gears, and rear disc brakes and bolt it up to your A body 8.25 rearend no problems!