The holes in the rear sill will give you the board spacing. There are several different board widths but use what fits between the holes on the rear sill since this is not movable like the crossmembers. The two boards that fit under the angle strips go in first and a hole is drilled through them at each crossmember end hole so both the bed sides and crossmembers are supported by these two side boards. Square the bed diagonally and lay the rest of the boards in place; they DO NOT fit tight together. Lay your strips in and if you have a horse shoe in your butt the holes will align with the holes in the crossmembers. There is a front and rear to the strips with the two closer spaced holes going towards the front. When everything is laid in place if the holes in the crossmembers don't line up--drill new ones.

Evan