Ah yes, of course, a "flatdeck" /forums/images/graemlins/thumb.gif
Fitting anything but a regular cab on your truck will take some work. The part of the frame under the cab area is about 4" lower than the rest of the frame. On a Suburban or crewcab the low section is much longer. If you place the crewcab body on your frame, it will sit too high... or to get it the correct height, you need to cut open the rear floor area, build your own rear cab mounts - then patch the floor together.
The crewcab is normally 165" wheelbase, but you could just shorten your flatdeck. /forums/images/graemlins/wink.gif