Hello, there were problems with this model in that the Dex Cool coolant would eat up all the aluminum components (radiator, heater core, water pump) You didn't get a lemon they pretty much all had this issue. There was a technical service bulletin (no official recall)
My suggestion would be to flush the system very well before installing the new radiator. You may also want to replace the water pump on the new motor if condition is unknown. I would also suggest using one of the newer dual compatible anti freeze mixtures. Prestone makes a great one that works with the red and green coolant.
The thermostat that I ordered thru GM is not working. Do I need to order another to replace that one or will one (less expensive) from an auto parts store work?