My truck (which is a '75 GMC) has a data plate riveted to the door frame, just below the striker post on the driver's side door frame. This plate has the VIN number, and it also lists GVWR (8400 lbs for my truck), and the front and rear axle weight ratings. This is supposed to be the stock data plate location for 73-78 GM trucks. Have you tried looking for a data plate in that location?
This URL shows a picture of a similar data plate (mine is a little different) http://www.chuckschevytruckpages.com/VINIDPLT.html