i'm not sure about the measurements of a stock beam.
how much you want to narrow will depend on the type of tires you run, and their offset, and how far into the wheel wells you want to tuck to achieve \"the look\". i would suggest getting drop spindles if you dont allready have them. they will drop the car and retain the stock steering and wheel geometry and ride. spindles usually push the tires out 1/2\" on each side, which will require some more narrowing. disks usually push the wheels out another 1/2\" each side.