coolant level low

Has any real work been done? ie: water pump , radiator ..... etc?
The water pump in your Northstar is at the rear of the engine, driven by the camshaft, rear being the drivers side

Yes, the water pump is not that old, the manifold was replaced first then they said it was the gasket at the thermostat that was leaking which should have been caught when they put the new manifold on but wasn't. They say that the threads to the bolts of the head gasket are stripped and that is where the coolant is leaking into the exhaust system. They say those bolts cannot be tightened or replaced the entire engine would have to be replaced. Is this true?

They have also done pressure tests that say everything is working correctly.

i see this almost every day , you do not have to replace the negine for the stripped (block threads ) it does need to come out and have time certs put in , that is where a jig is set on the deck of the block and all old head bolt block threads are drilled and replaced with time certs , no big deal on that part , but the negine needs to come down and heads removed and tell you the truth if you go that far have the case halfs selaed , lower oil pan gasket replaced and the rear main seal while its apart

I think I understand what you are saying-but I am not familiar with "time certs" the engine has almost 140,000 miles on it-would I be better off to replace the engine with a new factory engine or try to go this route? Thank you for all your help.

if the block threads have failed , you wil not see coolant on the ground , as far as replacement , if you plan on keeping the vehicle and can spare the change i would say yes go with a gm reman , it does have some miles on it and you may or may not have more problems in the future