Let the car cool down, take the rad cap off, start the car with it off, and look into the radiator. If you see bubbles coming up, it's head gaskets. Bit odd that it's not smoking at all out the exhaust. If it was head gaskets, there will be SOME type of symptom that you can easily see. Check for oil in the antifreeze, antifreeze in the oil, etc.

Are all coolant hoses on in the right spots? Is the steam tube line run to the throttle body? Did you replace the thermostat when you did the motor? Not saying these are necesarily causes, but you gotta make sure the basics are out of the way before assuming it's the head gaskets and ripping the whole motor apart. I'd change 20 thermostats before pulling heads...

 

What fasteners did you use for the cylinder heads? How did you torque them? What sequence did you use?