Just for the record. A 8-1/4 rear axle is good for 350hp and is an affordable option from the 8-3/4 rear. Just put one in my 66’ - a lot better than the smaller rear I had in there.
I’d question why rear disks on a car under 500hp? Rears provide only 30% (at best) of the breaking power of the car. They are there to keep the car straight under normal braking. I do admit rear disks look kool thru the rims.