WORD OF CAUTION - If you are using the Classic Auto Air (or any system that uses an electronic blocking valve to the heater core rather than a blend door) - You will need to install additional T's on the coolant line to the heater core path for a return to the engine when the heater is off, otherwise you will only have coolant flowing on one side of the engine. Another shop buddy was working on an E body and wondering why only one side of the engine was heating up. The coolant had no return unless the heater was switched on.
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That could have been bad. At least the lesson was learned without destruction (and not on my car / engine.)
