$500 for a used GT/V6 setup is a ripoff.
Here:
https://www.summitracing.com/parts/fms-m-2300-m
https://www.summitracing.com/parts/..._mmc=pla-msn-_-shopping-_-srese2-_-power-stop
Pick up a used pair of 94-95 Sn95 axles for $50-75 and some pads and that's a Sn95 COBRA read disk setup with NEW red powdercoated calipers for about $500. Other than the axles that would be a 100% new setup using the 11.65" Cobra rotors.
Used GT rear setup? $100-150 tops.
I run the above setup with 17x8 rear wheels and 255 width tires. But, you are limited on wheel selection due to the offset. There is a way to run fox width axles here.
What rims do you want to use?
EDIT: those calipers have an $40/ea core charge. I just looked on rockauto and with core, they are cheaper than summit, so I'd buy them there.