The original promotional models and kits from the 60's were long beds. They come up on ebay all the time, but are not inexpensive. Here are a couple of examples, the first one is just a box, but frequently a complete kit can be found in that price range:
http://cgi.ebay.com/SMP-1960-Chevrol...item5ada8be595
http://cgi.ebay.com/1960-CHEVY-APACH...item5d2bb250dc
The models produced as kits were made of styrene and normally don't warp. Early promotional models were made of some type of acetate, which can warp badly, so it's hard to find a decent original. Modelhaus made resin replica kits about 10 years ago, but the molds wore out and they dropped them. You might find one on ebay with enough searching.