Makes us wonder why it was sitting so long.
Yes, that will definitely cause it to overheat. If there is combustion gases getting into the coolant system, no amount of bleeding is going to work. A cylinder leak down test can reveal if there is a head gasket problem, cracked head, or cracked cylinder wall. You'll see bubbles in the coolant system when you pressures the cylinder. Compression tests may not give you an indication if the leak is small.
Some of techs use vacuum coolant lift system to do refills on those, because they do like to trap air sometimes.
Here is the service procedure:
FILLING PROCEDURE
1. Close the radiator drain cock.
NOTICE: When adding coolant, it is important that you use GM Goodwrench DEX-COOL@ or HAVOLINE DEX-COOL@ coolant. If Coolant other than DEX-COOLS or HAVOLINE DEX-COOL@ is added to the system the engine coolant will require change sooner-at 50 000 km (30,000 miles) or 24 months.
NOTICE: Do not use a solution stronger than 70% antifreeze. Pure antifreeze can freeze at -22°C (-8°F).
NOTICE: This engine uses DEX-COOLS and GM coolant supplement (sealant) P/N 3634621 specifically designed for use in aluminum engines. Failure to use the engine coolant supplement (sealant) and the approved coolant antifreeze could result in major engine damage.
When refilling the cooling system, add three pellets of the engine coolant supplement sealant GM P/N 3634621 to lower the radiator hose.
2. Refill the cooling system. In order to ensure sufficient Cooling System, freezing and corrosion protection, maintain the coolant protection level at -37°C (-34°F) or lower.
3. Set the heater and A/C control in any A/C mode except MAX. Set the temperature to the highest setting.
4. Allow the engine to continue idling until the lower radiator to coolant pump hose becomes hot.
5. Cycle the engine speed up to about 3,000 RPM and back to idle 5 times. On the VIN 1 engines, slowly open the bleed valve on the rear of the thermostat housing for about 15 seconds in order to release any trapped air in the cooling system.
6. After the air has been released, fill the radiator. Install the radiator pressure cap/surge tank. The arrows on the radiator pressure cap must line up with the coolant recovery tube.
7. Allow the engine to cool to the outside temperature. Check the coolant level. If the coolant level is not at the Add (of Full Cold) mark, add coolant until the coolant reaches the mark.