Heater heat

My 68 has a control valve on one of the heater core hoses since it is an AC car? Does yours? Is it opening and functioning correctly if it does?

What temperature of a thermostat are you running and is if functioning correctly?

Do you have an infrared gun to take readings on different locations on the system to see where things are failing?

I have used CLR with excellent results in the heater core to remove deposits. I remove the hoses from the engine and use compressed air set at about 15 pounds to remove the antifreeze and water. Then I pour the CLR in the hose that has the connection the highest on the heater core. I let it stay in for about an hour and then flush the heater core out really well from both directions several times.

This is a 1967 A-body valve.