The 8.5 is worlds above the 7.5 rear. With the increased ring gear diameter, the gears are also thicker and wider (contact patch is larger). With nothing more than C-clip eliminators, stock factory geared 8.5s in the turbo regals are running mid to high 9s in the TSM class. With stonger gears and axles they'll run deep into the 9s.

 

I know the GNX's rear ends were with a PHB/TQ arm setup, not a 4 link or leaf spring setup. They made it a torque arm setup by mounting the torque arm to a different rear cover. What I am wondering is if it would be possible/cost effective to convert an 8.5" rear for our 4th gens by swapping axle tubes/custom axles, and using a GNX rear cover with a custom torque arm?

 

Cal Hartline's GN is into the eights with his 8.5 10-bolt. Theres a video of his car with the wheels in the air at half track.

 

I have one video of him from a couple years ago.... 8.65 at 155mph, launching off the transbrake and walking the front wheels in the air the first 200 feet of the track. Full weight, factory interior (except the front seats, obviously), AC, heater, sound system, daily driven... that car is sick! Last i heard he's getting awfully close to running 8 flats. Reminds me a lot of Matt Harlan's firehawk.

I'm pretty sure he's running a spool in the rear though. There's some point along the ET ladder that rules require spools, IIRC. I'd be amazed if the GM carrier can handle those stresses.