The vin decoder you used is for passenger cars. The truck VINS follow a different format so that decoder doesn't work.
VIN decoders aren't likely to provide much more than what the service manual image above provides. When I checked on my truck back in the '90s folks at the Heritage Center said GM didn't keep track of trucks like they do now, and they lost many of the records in a big fire in the '60s.
Here's a neat page, mostly for older trucks. It has some cool body options which may help in your quest:
http://www.gregwapling.com/hotrod/ch...dian-chev.html
That website also mentions contacting GM of Canada to obtain a restoration packet.
"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 "