Easiest way with the old cars is to just put the G500 springs on from the modern car, it raises 1.5" and is evenly set, and these springs are easier to obtain.
You must stick with a progressive diameter coil spring on the rear......fronts don't need to be since axle weight does not change as much on the front with varied loads vs rear.
You also need to get a new brake bias adjust spring, and adjust the linkage between the spring and the axle.
Otherwise the springs change out easy, remove shocks and loosen arms drop axle, and go