rear end question

I think that general rule of thumb applies to the vast majority of A-bodies but apparently there could have been some variations because according to the fender tag on my 68 Cuda it was a drum brake car all around, 318 engine, yet it had a 727 tans. and 8-3/4 rear. It was built like a base 340 car but only with a 318. I wonder if there was some heavy duty suspension package available that got you all of these? Nothing noted in the fender tag and I don't have the build sheet but it was just a thought..