US Chevrolet Restoration Packet (free)
This is the ultimate reference material for your truck. It includes information regarding vehicle weight series, dimensions, colors and optional equipment etc. I believe you can order it through their website . Just go to their customer service page or by calling the 1-800 number listed below.
NOTE: You will need to quote your VIN number
Chevrolet Customer Assistance Center
Chevrolet Motor Division
General Motors Corporation
P O Box 7047
Troy, Michigan 48007-7047
Phone: 1-800-222-1020
Canadian GMC Restoration Packet
The GMC is a little more difficult to obtain information for some unknown reason. I've found this Phone number 1 (800) 467-6853 (GM Canada Vintage Vehicles) to call and receive a similar Restoration Packet as the Chevrolets. This packet supposedly contains information on (trim codes, paint codes, body numbers, plant of manufacture, engine size etc.,).
Unlike the Chevrolet Restoration Packet which is Free, the GMC Packet Cost is $25.00