This website deals specifically with GMC trucks.
http://www.oldgmctrucks.com/ They have a Discussion Board that you can join.
I'm not sure but your truck should of had a number emblem on the side of the hood above or below the GMC emblem.
Something like 100, 150, 350 would give you the model/series tonnage of the the truck.
The tag on the drives side hinge pillar should give you the year/model/series of the truck.
oldgmctrucks.com website is kind of hard to find stuff but just keep clicking on links and you will eventually find more info..
ID page
http://www.oldgmctrucks.com/page0012.htmlGMC Serial number page
http://www.oldgmctrucks.com/page0010.html281 22 = series 280
http://www.oldgmctrucks.com/page0015.html
