my 75 - L82 (non air) car had a factory vacuum shutoff that closed a valve to the heater box in summer when you moved the outside air control to the max position, I installed a 4 core rad with larger passage tubes and my overheating went away which made the car much more enjoyable in the summer heat, alternatively you could try a pusher front mounted electric fan but if you tubes are plugging off it won't help much.. My guess is that the heater core was either plugged or leaking or both and that is why it was removed, if that was the case your rad is likely partially plugged also. One long run we took in the summer to the Corvette plant we all took our headlamp bezels off and drove with our headlight doors up and the hood popped open, that seemed to help also. We all had C3's back then, great cars!