I very much doubt you'll get a proper CAD file of a vehicle. The nearest thing I've found is 2D sketches in Upfitter information provided by manufacturers for companies building bodies on their truck chassis. 1994 is a bit too far back, but this GM site goes back to 1999 which may be the same body - as I'm a European, that's more knowledge than I have.
GM Upfitter Body Manuals

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