Don't automatically jump to the conclusion that the head gaskets are bad.
The Northstar engine self purges any air in the system as long as the purge line is clear and flowing. Have you checked that? With the car cool, disconnect the 1/2" line at the surge tank that comes from the throttle body area. Remove the surge tank cap and place the disconnected hose in the neck of the surge tank. Start the engine and observe the flow from the purge hose - there should be a steady flow of coolant peeing out of the line. If not, you have discovered the root cause of the overheating. Follow the hose back to where it connects to the water pump crossover (it passes through the throttle body first) and disconnect the hose, remove the hollow bolt and clean it out.
Another possibility is your radiator cap is not holding pressure. Very easy to test it with a borrowed tester from Autozone.
Rule out the easy things first.