If you're set on the S-10 diff, in 1989 they started using a bigger unit, 7.625" if I recall. They weren't on all '89s, just started appearing then. They're not easy to tell apart from the 7.5" just by looking at the cover. You'll need a maximum of 4" backspacing on your rims to make the tires clear your bed.
I've heard a '72-ish Nova/Apollo rear axle is pretty much a bolt-in. I would bet that's a beefier unit, and available in a number of gear ratios. Get the speedo gear from the donor vehicle while you're at it, it might help down the road. '89 S-10s used an electric speedo so the "gear ratio" is stored in the ECM.
