Cost for A Body 8.75 Rear End

I've had good luck with my 8.25. Originally had 2.45's. Then upgraded to 3.55s with auburn sure grip. Held up for 15 not so delicate years. Now went with 3.91's same rear. However, I guess my car is mostly street not any drags! 360 and the loved and also hated 292/508 purple cam is the powerplant. Not so serious I would guess.