PDW, it depends on your cross sills. The important thing is the stacked up height of each cross sill and its wood blocks should be the same height as the rear sill mounting brackets. Original cross sills were a little over 1-7/8" tall including the bracket and the rear sill bracket is 4" tall. Typically 2-1/8" would be a starting point, but measuring on your truck is best. The bed should sit level on the frame and all mounting points equally share the support of the bed when installed. If your frame is tweaked so the bed to frame mounting points are not all exactly in the same plane, you may have to adjust some points up or down to accomplish this. The sketch in this instruction
http://www.mar-k.com/bed_mounting_information.pdf will illustrate the idea.
Mar-Ktech