Check this out-
http://www.oldgmctrucks.com/page0010.html I know it won't help a whole lot but gives some clue as to why it's so hard to nail a year down on old GMCs. It looks like you have a '39-'40 1/2 ton long bed with a 228. That doesn't help much on the year now does it?
By the way, I love your '39 Chevy!
