You don't want the weaker 7.5" rear end to replace your 8.5" rear end. If you want to replace it go with a Dana, Ford 9" or if you just want disk brakes you can go with a Impala/Caprice rear end. You may have to get spring pads welded onto the Impala rear end to work with your truck and if you have 6 bolt wheels you'll be out the cost of buying some 5on5 truck wheels.