Chevrolet Repair: 350 chev in 1981 CAMARO, radiator hoses, 1981 z28 camaro

Question
Hi, I have a 1981 Z28 Camaro 350  with turbo 350 auto trans. I have over heating problems with it only when driving at high speed down the highway. It doesnt get hot sitting at the lights in the city, only when driving at a high speed for longer than 15 minutes. I  have installed a brand new 3 core radiator ,checked the water pump for flow rate, replaced thermostat to a lower temp, checked temp guage for accuracy, checked the clutch on the fan works fine, replaced top and bottom radiator hoses and replaced the heater core and all new hoses. So anyways it still gets hot going down freeways and when I pull over its obvious that its overheating as because the water is bubling and flowing out of the water overflow container. I have also replaced the radiator cap. A mechanic tells me he thinks it might be a broken head gasket even tho there is no water visible in the oil or oil in the radiator water. I have not yet done a compression test to establish if the head gasket is broken. what do u think it could be as Im getting a little frustrated and am hesitant spending a wedge of cash getting the head gaskets replaced, and how much do you think it would cost to do the head gaskets?

Answer
What temperature thermostat do you have in it?
Is it the correct one?
If the motor is supposed to operate at say 200 degrees and you put a 185 degree thermostat in, the thermostat would never close enough (at high speeds) to cool the system properly.