The 10 bolt you have should be plenty strong enough for the HP you'll be putting out. I would just pick up a limited slip unit for it and go with that, up to you. A ford 9 inch is a very good rearend but could also cost you alot of money too, 1200 dollars isn't out the question for a well built 9 inch. I have about a 1000 dollars in my chrysler 8 3/4 inch, and got the housing for free and the limited slip chunk for 150. If you go with a different rearend than chevy built you will likely need to have it cut down to fit your car, buy new axles to fit and all of the other parts that go inside such as gears and bearings, it adds up.
If your lucky you might find a good 12 bolt for cheap if you can find someone with one just hanging around. Even if you need to get a Limited slip chunk you can come out ok.
Take a look in some performance catalogs for gears, axles bearings and such for rears and the price really adds up quick.