1976 Dart Overheating and other issues

I would first take the radiator out, and take it to a radiator shop, get it flushed out properly, possible it even needs re-cored, it looks really rough on the outside, the inside cannot be any better.

Second, the factory gauges are not too accurate I would add an aftermarket gauge, on my 76 duster, it was a simple as pulling a plug above the thermostat housing, and adding a mechanical gauge.