Roger55, here is some part link info and alternative drum info from the archives in 6066 GMC site. Hope this helps.
http://www.autopartsnetwork.com/1226...q7KrRfpTt5kdHI
Amazon.com: Centric Parts 122.66037 Brake Drum:...
Matt: The front drums are easy. They are the same as a Chivy and should be easily found and are relatively cheap. They have a 3" height or depth. Here's Rockauto.com's listing.
RAYBESTOS Part # 2005R $33.79
The rear drums are a bit harder, but I have three part numbers you can try for rear 6 lug brake drums. These have a 3.5" height or depth. The first is CENTRIc Part # 12266037 . The second is Allsafe 2067 DGS or Raybestos 2067R.
Rockauto.com shows the Raybestos 2067R for $58.79. Rockauto.com also has the Centric Part #12266037's for $119.79.
I used the Allsafe 2067 DGS from Autozone. The 2067's are for a later model (i think 67-72) truck with 4 wheel drive. It has fins on the outside unlike the original that is smooth. The only modification that I had to make was to drill the 6 lug holes from 1/2" to 9/16" so that the drum would fit on the shoulders on the rear lugs like the original. This drum had the right offset unlike the Chivy drums.
On the Centric, I haven't used that one, but was told it was a direct match to the original (a bit pricier though).
I hope this helps with those looking for rear 6 lug drums.
Regards, Steve New Orleans