400's are they any good?
Over at BBD.com they have a list in the tech section of what the weights of steel vs. fiberglass parts are with a total savings of weight to the right of the list. They show a good possible weight reduction for the A bodies.
Know your cars actual weight before making any calc's and do not go by the reggie's listed weight. Drive it onto a scale if possible (At a junk yard?) and ask for the weight of the car. Also, realize the amount of gas in the car in as close to exact gallons in the tank. I forget what the weight of a gallon of gas is. 7#'s per gallon? I do not remember. But minus that from the cars weight.