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
