Heater Box Design - Lame?

I recently rebuilt my 1969 Dodge Dart non-AC heater box. What fun! I always thought a valve shut off the hot engine water coming through the heater hoses, through the firewall, and into the heater core. I learned there is no shutoff. Therefore, the interior of the car is being heated full time (the interior heater box is made of thin uninsulated fiberglass, 3 flaps keep most of the hot air from exiting the heater and defrosted ducts), even in summer. Today, I measured the exterior of the box to be 120 degrees, probably goes up to 180. Hot inside the car, even with the windows open. Am I crazy, or is the heater box design lame? Has anyone ever installed a shutoff valve in the heater hose for summer driving? The heater core probably does help cool down the engine somewhat. Would a heater hose shutoff valve impact engine cooling? I know this is 50+ year old technology, but WTH?