I have mounted two different older campers (1987 and 1994 campers) on two different late model Ford trucks (2003 and 2010).
Measuring the bed rail height is a piece of cake. Measuring the cab height is difficult. Cabs tend to have curves or rounded surfaces, and, the high point you want to clear is well forward of the bed surface that you measure from.
What worked for me was using "temporary" spacers: 2x4's on edge, old pallets, cinder blocks, whatever I happened to have on had at the time. Just load the "spacers" in the truck bed. Lower your camper onto the spacers. Then measure your cab clearance.
After you figure out how much cab clearance you want, add or subtract the difference between your actual, measured clearance and your desired clearance and add or subtract that difference to/from the height of the temporary spacers to calculate the height of permanent spacers.
