If you choose to go aftermarket as I did with my '68 Lemans, I went with the Ford 9". My reason being is that the 9" has been around since 1957, and cars from HP to street rods to drag racers have been using them ever since. Aftermarket parts are plentiful and many build their own housings to your specs.
I chose Quick Performance over Moser, Strange, and others just because they seemed to be less expensive, but in the end, the rear end assembly I pieced together as I wanted it was probably no less expensive than Moser, Strange, or any other.
Quick Performance Ford 9 Inch Rear Ends
had the housing which I added a few upgrades looking for a rear end that would handle 600HP without issue - not that I will get that out of my 455, but I wanted a rear end to handle the torque, a shot of nitrous, and wide 12" tires grabbing the pavement. Of course a rear that will handle this kind of HP/torque is useless if the suspension/frame are not beefed up as well - which I have done.
The real beauty of the 9" is that it has the removable center section so if you want to play around with gear ratios, you can actually set-up another center section and swap it out - like a gear for local fun or a set for long trip highway cruising mode.
I have not tried it, but it is under my frame and I did not experience any problems. I opted for the Quick Performance 11" rear drum brake set-up which I felt was more than enough with the front disc brake conversion I have added. Again, I mixed and matched all my center section parts then had a local guy assemble it for me - $125.00 which to me was a deal.
I did encounter a problem using the factory type rear sway bar. It will not fit as the rear center housing is larger than the 10/12 bolt. Companies like Spohn make a rear sway bar that will work - but more expense. Not going original by any means, I created/fabricated my own means to attach the factory sway bar going out the back of the rear axle/frame as some newer trucks use.
I believe the driveshaft will have to be fitted, but the universal joint size is the same on Pontiac & Ford. Not a big deal with me as I'm not going original and expect these things during my build, so you want to inquire as to driveshaft fitment with whatever rear axle you go with.
So do a little research on prices whether a complete turn-key rear axle assembly that will bolt under your car or if you want to piece meal it like I did. Also, options on the type of posi you can install. Some can be more noisy and harsher in locking up if you go with a "locker" style unit. Piece mealing mine together I went with a Power Trax locker https://www.powertrax.com/product-in...action-system/
Again, this was my preference and the 9" will be around for a long time so parts will be available, they hold up, and can be a selling point should you ever want to sell. It is not inexpensive by anymeans, but I felt I only wanted to do this once and wanted it to last my lifetime.
BUT, as Pinion head has stated and made mention of his work and the 8.5 10-bolt, this would probably be the best bang for your buck, most inexpensive route, direct fit, and capable of handling your HP. I have a friend with a BB Nova doing 10 second 1/4 mile times with an 8.5 and he swears by them. Says the ring gear will wear over time, but the rear itself holds up to the abuse.
You can often find these on Craigslist for $100-$250 and then build it up from there.