Now since I found out I had an 8.5 I would use it. Before I got this rear end my daily driver a 04 silverado's rear came apart. I put an eaton posi in it, but I didn't like in the truck. So found out that eaton posi will fit in a 8.5 if you change the carrier bearings and its 30 spline.
Wow... That looks like quite a project that you have had going on in your home shop. It looks great so far.
The way you posted with latest pictures and then you worked backwards...
Quite an accomplishment...
I actually got something done on the car and have new pictures. I ordered a cold air intake put it on and hated it. So I took it apart and used most of the stock GTO stuff. One thing it put the MAF way to close to the throttle body. I also put some of the exhuast on. I need to get tail pipes on, but I can't find anyone willing to touch it. One guy told me he would try, but it would be like $4000 and look like sh#@.