There is a couple clues in your horror story that indicate head problems either gasket, warped or cracked head 1. water in the oil (milky white) and high pressure getting to the cooling system. The thermostat allows heated water at the thermostats set point to go through the radiator. If it hasn't reached that temp the thermostat will not open. It will take about 5-6 min to get to that temp, so at the most your thermostat is probably just starting to open.
Now i am testing my memory, but I believe there is no overflow system on the 74 and the radiator cap may be venting the extra pressure/coolant out of the vent line on the cap.
If you have a pressure tester you can check by placing it on the radiator instead of the cap and start the car if pressure runs up quickly it is defiantly something with the head. (best done with 2 people incase you need to turn the ignition switch to off quickly.