A GMC has five freeze plugs down the driver's side of the block. Pull the intake/exhaust manifold assembly and remove those plugs. That will give you a clear shot at the bottom of the water jacket for a detailed cleaning. Replace the press-in plugs with the rubber expansion type that bolt in.
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!