Just curious. If you're considering going with the Dana vs your stock 8 3/4 I would assume you're setting the car up for some serious HP, otherwise the original rear would work fine. If that's so, I'd personally order the Dana in B Body length, order the Dr Diff 1" offset rear spring hangers, and put on 9.5 - 10" rear rims & wider tires. You will want to call Firmfeel about using the E Body rear sway bar with the narrower rear, but I've read elsewhere here that it should still fit. You may have already considered all this so if so .... nevermind!
Later, Jim