You asked the easiest way to do the rear end - that would be to find an A-body 8 3/4 that bolts right in. As has already been pointed out, they are not easy to find, and usually command a premium price.
At the low-buck end and still pretty easy end of the spectrum, you could get a Ford 8.8 (late 90's Explorer is ideal) from a boneyard - they have a 5 on 4 pattern. They are relatively easy to shorten to an appropriate width (tons of videos on YouTube), and you can get hunt for your choice of gear ration, find a limited slip unit, and get them complete with disc brakes.
