Additionally, we thought it was a head gasket as we had narrowed it down to coolant entering the cylinder, but alas after pulling the head, the gasket was fine and prior to the compression checked out. His head was cracked due to running low on coolant when the resin water thermostat housing cracked and seeped coolant w/o his knowledge... (another thing to check)
Good luck on your search, I would probably as a next step do a coolant system pressure test to begin with, but keep in mind, in the case of cracked components, they don't always show on a pressure test as they do not leak until they expand when hot. I had a 93 Bronco with a 351W that the PO did not run the right coolant, it lost coolant for a while before I found one day it spraying a fine mist out below the spark plugs on the pass side... the head mustve froze and it was cracked end to end in many spots... Then I was able to notice the fine green lines where coolant had been leaking and drying over the thermal cycles... dont rule anything out I guess
JM2C