[LS short box]

Can't speak to the overall strength of the stock 12 bolt. On my 68 I used 30 spline axles from Moser and a TruTrac posi. Motive 4:10 gears. I was running only 400 rwhp.
But if you want a 9" I would contact the folks at Quick Performance in IA. They seem to have a good rep.