What happened to the old gas when it was sitting? Reason I ask is because the old gas turns to varnish. The motor will run fine the first time started in years, but that varnish sets in on the valves and causes them to stick when the engine cools down.
My guess is you have a few stuck valves and bent pushrods. Pull the valve cover off and have the engine cranked over to make sure there is movement in all of the rockers.
If you have a few bent pushrods, you can straighten them with a hammer and throw them back in. Those GMC motors were rugged as all "H -E- double hockeysticks. About the only things known to kill them is bad gas and high heel shoes!
John
