You nailed it when you said a tape measure is your friend. On "orphan" body/frame swaps we have found that after first getting BOTH old and new frames stripped and spaced to the desired ride height one can run a center line length wise down both frames to pull all measurements from. The constants are the front and rear axle center lines since you want to end up with the wheels/tires centered in the wheel openings. The first measurement will be for wheel base to see if or how much the frame will need to be shortened or lengthened. Once wb is correct then the location of each bracket, mount, etc is measured from the front-rear and center line of the OLD frame and then positioned at this very same location from the F-R and center line on the new frame. If precise tape measure exercises are used the body/bed/core support etc bolt holes will line up with no "wallowing out" required. Brackets will have to be fabricated, cross members added or removed, custom steering column drop made, and all the other joys of messing around with the General's perfectly good design but this pays us to keep the lights burning. Good luck!
