Funny that they would still use the 9", too. It has a deep hypoid gear set and generates a lot of heat due to the sliding action of the ring and pinion. The cars run coolers that circulate gear oil through the rear. At least the ones I've looked at close up do.

A modern quick change type rear would allow the driveshaft to be just as low, but run cooler due to a just having a spiral bevel gear. I think NASCAR doesn't allow them on cup cars, though.

Sorry, guess this is getting OT of the original post.