Most likely your rear is a 7.625" 10-bolt. Its possibly an 8.5" 10-bolt. The 7.625 isn't particularly strong, nor is it particularly weak. At 400 hp, I would seriously consider finding an 8.5 from nearly any HD B-body except wagons. (wagons used a slightly wider axle and you might have fender rubbing on a non-wagon car) You can find them on many Taxis, cop cars, and all Impala SS. If you plan on passenger car tires in like a 235 width, they will lose traction before the rear end blows, but if you plan on 400 hp and getting it to stick to the ground with some performance sticky tires, make sure its an 8.5"
If you want, find a late model B-body with the disc brake rear. Its an 8.5. I can even tell you how to modify your stock proportioning valve to properly operate the rear discs.
Here's a website to help you identify your axle. The site lists a 7.5" which is the same case as the 7.625" If yours looks like a circle, its an 8.5. If its slightly squared off, its the smaller 7.5/7.625" Scroll down to the GM part of the page:
http://truckparts-specialists.com/ca...ture-guide.htm