Advice removing heads from 360

Shall we go over this head gasket problem, so maybe you don't do this for nothing?
Sure.

Engine is running a bit rough, overheating. Removed thermostat and tested it, opens correctly. Checked coolant, appears to be flowing correctly. Attempted to remove air from the system using funnel. Exhaust gasses continuously coming out of the coolant system. With the cap on, the coolant system way over pressurizes.