Mike,
I ordered it from GM Vintage services, It cost me $57 (i think). When you call them, they fax you the form to put all your info on. I had the package in about 4 days by mail. They didnt have the brochure detail for GMC but included the 1970 stuff for Chevy trucks.
Here is the link to the web page:
http://www.gm.ca/gm/english/shopping/parts/vintage
Gaylon