Do all A bodies have overheating problems?

Wondered about the thermostat because my newly rebuilt 340 with new 185' thermostat heated badly. I pulled the thermostat and placed it in a pan of water on the stove. I watched carefully, and it was at a full boil untill it opened which was not right. I replaced the thermostat and it has not heated since. If the thermostat is okay I am at a loss to help if other solutions already posted don't work.
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