I put a head gasket on wrong when I was in college on a 351W. It blocked rear passages of the head. Never showed hot, but over 18 months it cooked the rear cylinder. Unless all passages are blocked I can't see it showing hot on the gauge unless gage is reading off rear of the motor.
The gauge does read from the rear of the head and the rear passages are the ones the have the tendency to get plugged. Does the rear engine block drain petcock (located on the drivers side) flow fully?
