here is an interesting article on rear ends http://www.chevyhiperformance.com/techarticles/0304_nine/

It is just my opinion but...

Unless you really going to be producing lots of torque the 7.5, 8.2 and the 8.5 ten bolts are sufficient. For a 305 to a 350 producing up to 400 Lb/ft of torque the 7.5", while small, is capable with a better gear set. For up to 475 Lb/ft of torque then the 8.5" tens are really nice. They are only .25 smaller than the 12 bolts and can put up with mid line torque. Now I am assuming were talking about Automatic Transmissions. If were talking about standards then this changes and the 7.5 and the 8.2's are not that good of a choice. Still the 8.5 is. And again, the Rear end Ratio has a lot to do with it. a 5.88:1 would not be a good choice for a 7.5" or and 8.2" it would be weak.

I put a 4.11:1 in a 7.5" but I also know that it will go bad if I keep it in long enough. I will, one day, upgrade to a 8.5" Rear or possibly a 12 bolt some day. For a small block with an automatic transmission the 8.5 10 bolt and the 12 bolts are good up to a limit. I woldn't consider them to be a lasting thing in a really stout small block with a standard.

If strength is a real issue then neiter the GM 12 or the Ford 9" is a consistantly good choice. The Chrysler 12" (Dyna 12s) are a much better decision. But they do weigh in at a heavier price tag. You'll only buy one with one set of gears and that is all you'll need. This is where we're talking big blocks and standard trans say a T56 or a Linco (sp). And then 700 hp and up and 700 Lb/ft and up of torque. I'd never trust a GM12 or a Ford 9" for that type of consistant power.

Again it is just my opinion. FWIW :D