I also have a 71 Cheyenne short box. I would use the spindles to lower it. I have spindles and air bags and running 20's. I have 20x8.5 on the front with 255/40R20's and 20x10 in the rear with 295/35R20's. The fronts are a 5" backspace and the rear is a 6" backspace. You should be able to get away with a 5.5 to 5.75 back space on the rear wheels. Here is a photo. Good luck and make sure to post some pics.
Also, Welcome to the forum.