The rear end I'm planning to go with is a '71 -'72 Buick/Olds A-body 8.5" corporate 10 bolt. This would be a direct bolt-in on your car as well, the rear U-joint is a little different than the one that came stock on our cars if I'm not mistaken.
These have the tapered axle bearings that are said to handle side loads better along with a true heavy-duty axle. They are 28-spline axles that are bolt-in style instead of C-clips like most GM 8.5" corporate 10-bolts. The factory posi units are fine as long as you're not drag racing the car with any kind of traction tire. These 8.5" rear ends are nearly as strong as a 12-bolt 8.875" rear and share the same pinion bearing.
I'll upgrade my 8.5" rear end to a new 30-spline Eaton posi unit and new Moser axles. Part of the reason being I need to change the bolt pattern on my axles to 5 on 120mm to match up with my BMW-spec aftermarket wheels, so new axles are the easiest way to accomplish that.
The stock GM intermediate pattern is 5 on 4.75" (5 on 120.65mm) so there's not a lot of difference between the two, but I want things done right. Upgrading the axles and posi unit isn't a bad idea no matter what, just more $$. This way I can beat on it and not worry as much.
Spend it only once and do it right whenever you can.
