Just get an 8" rearend out of a 4dr car in the junkyard. You might as well get the front disc brake setup at the same time. You're not going to hurt an 8" rearend w/ a 302.
If you're going to spend the money on a rearend swap including a posi unit, you may as well go with an Explorer 8.8 (and narrow it) or narrow a 9". Either one will set you back $600-800 after you're done. The late 70's to mid 80's Bronco 9's are the easiest way to a traction-loc 9" rear with 31 spline axles. All you'll need to layout in cash are the new axles and brake drums, if you leave the center section as-is. Plus labor to narrow the housing.
