The weight difference between a Dana and the 8 3/4 is not that much. Especially if you're running a spool.
8 3/4 and a Dana set up exactly with the same gears both with a spool in the same car no other changes. The Dana will be faster because it takes less power to turn the gears in a Dana than it does an 8 3/4. Parasitic loss I believe that is what it's called. Same reason a 904 is faster than a 727.
It is cheaper to build the dana than an 8 3/4 and it is much stronger.
