As per my post of 5-24-12. Did you ever check the temp with a known good guage?
Correct me if I am wrong, all air through the core is from the vent. Hard to beleave.
You said turning the heater on or off had an effect on engine temp. Are you refering to the fan control or water valve?
Back in my day (do I sound like a TV ad?) and when that Rambler was built we didn't have a Vacuum sucker for the cooling system.
I still think you have a guage problem not a cooling problem.
Conferm true engine temp and we'll talk.