One way you might identify the problem head would be to pressure test the cooling system overnight or repeatedly with the sparkplugs out. Then crank over the motor and look for coolant coming from a cylinder.

Then you should do both heads as it is 2/3 of the work to do just one head and they both fought in the same war.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician