Most measure form wheel mounting surface, out side of drums, to wheel mounting surface.
Use one of the online gear selecting calculators to find the best gear ratio for your transmissions and tire size. With overdrive, you have plenty of choices. I would guess 3.73 would be about right with a 28" or 29" tire.
To set the spring perches close to start, measure the old pinion angle compared to the old perches. Level the perches with bubble level, then measure the pinion angle. This should get you close. If you plan to lower the truck, then just clamp the perches in place with the u-bolts, finish up the chassis work, and set the pinion angle very last thing once you have all the weight of the truck on all four wheels.
Always remember, the pinion angle needs to be set when the truck is on the ground, all four wheels on, full weight, and driveshaft in place.
Joe
