I would say if you were doing some kind of special deal where you HAD to use the original rear for the era...chop away. Is this origianl Grand National car? If not then go fo rhte gusto. Again, YOU must do what you feel is right for you. I am just here to give other opinions. Price out all the stuff you think you need for the 8.5 plus the cost of the 8.5 labor to assemble it. see how close you come to the 12 bolt moser rear. then PM me and see what I can get it done for. Ok? Jim