How to Measure for the correct back spacing?

Very easy to figure out. Jack the car up and put stands under the axle. Take a rear wheel off and place a straight edge across the front of the drum on the flat part that is sandwiched between the wheel and axle hub. Measure from the straight edge to the leaf spring and that will give you what you have for space. Now don't forget that you have to account for tire bulge and also spring flex. You should have a minimum of a .5" between the tire and the spring.

Hope this helped