Tire conundrum

I am having a similar problem I have 15x8's with 255/60/15 tires 4.5" offset. My rear springs are saggy and I really only will need 1 " in height to cure the rubbing. My problem is I don't know how much my springs have sagged in 40+ years. Does anyone know the distance between the axle and the frame rails on a stock 1967 Barracuda with fresh stock type springs? I can then tell if I only need stock or +1" spring like the XHD's. I just don't have the money to "experiment" with 2 different sets of springs. Right now there is about 1" between the tire and lip of the fender. I may roll the fender lip to solve this but it still leaves the saggy rear as compared to the front.