I don't see any reason for a posi, just more things to go wrong, and if you have a transmission with an overdrive 5th, why bother with the trouble and expense of changing the rear? a 3.90 rear is plenty 'torquey' and a .7 high gear drops the rpm enough at high speed, the small blocks like rpm a lot more than lugging .... and if you have to remove/replace a crossmember, as when installing an automatic, I'd weld the new one or use grade 8 bolts, otherwise what rivets would you want to remove??
Bill
