Using 8 3/4 with /6

You will be looking for an A body housing and axles. They less common these days so many people buy B body or C body diffs and get them cut them down at an engineering shop. Axles can be shortened or bought at the correct length new.

You will need to get your driveshaft shortened. You could also consider getting a whole new bigger one if you are going to make more hp in the future.

New shock plates. New U bolts.