DO NOT put gas into the onboard fuel tank. Use a remote tank like one for an outboard boat motor, etc. Fresh gas will dissolve all sorts of crud left over from decades-old gasoline, which will end up in the valve guides. The next time you try to start the engine, you'll probably bend a bunch of pushrods and/or rocker arms, or even hit the stuck valves with the pistons. It's a chronic problem on Chevy engines, and probably the same thing will happen on a GMC.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!