Have you made sure the engine isn't fighting a rusty spot in one or more of the cylinders or some tight valves? NEVER, ever, assume anything when trying to resurrect a long-dead engine. If you haven't been soaking the cylinders in penetrating oil and you've made sure the engine turns freely by hand, stay away from the starter. Drop the inspection cover under the flywheel and use a short pry bar on the flywheel teeth to turn the engine by hand until any tight spots go away. This might take some time. While you're at it, remove the valve cover and soak the valve springs and stems liberally to avoid problems with sticky valves. Those GMC parts are scarce and ESSSSSPENSIVE- - - - -don't damage them by trying to force the engine to turn past a tight spot! My favorite rust buster is KROIL.
www.kanolabs.comJerry
