This 1970 kit is the earliest I have seen in a while for that series of truck:
http://cgi.ebay.com/1970-Chevrolet-F...item4cf7818985
There are usually some promo models available on eBay, like these 1968's, but they are usually priced beyond the casual hobby budget. Also many of the early promos were made out of a plastic that warped badly.
http://cgi.ebay.com/1968-CHEVROLET-C...item5d2e140bad
http://cgi.ebay.com/1968-CHEVY-FLEET...item5d2e1469aa