GMC radiator petcocks are at the top front of the radiator.
When you fill the radiator have this petcock open, when water runs out of it, stop, it is filled as far as it should go.
Close petcock.
Some coolant will usually run out no matter what you do.
180 degrees is too hot, these trucks and older farm tractors use 160 degree thermostats which
are readily available at your friendly auto parts
store.
