I had the same problem with an Australian musclecar some years ago. Same symptoms - would push out fluid after driving the car, but didnt run hot. Car idled fine and didnt push fluid outwhen idling. We did the dreaded C02 test by removing the radiator cap and there was a small amount of carbon monoxide in the cooling system. Took the heads off and they required decking (milling), were very slightly warped. Car ran perfect afterwards.
Do the c02 test to see if there is carbon monoxide in the cooling system first, then could be just a dud head gasket but get the heads checked for trueness while they are off.