engine that is overheating?

I have the car listed above engine that is overheating. I have pressure tested and confirmed there is no loss of coolant. I have removed the thermostat and bench tested to verify it is operating normally. I removed both hoses from the radiator and completely flushed it with a garden hose to verify the radiator has no debris and water is flowing through it. I did a combustion leak test and there is no change in color of the fluid. I also replaced the timing belt and water pump as the belt seemed stretched slightly and I suspected it was not turning the water pump. I verified both fans are operating on both high and low settings. While the vehicle is overheating, I can turn the heater on, and it will lower the temperature to normal operating temperature. I’m kind of stumped at this time as I have completely ran through the cooling system and not seeing anything out of the ordinary. I would like to note I’ve burped all the air out of the system as well...