1973 Dart Swinger cooling

Based one your analysis of the situation, it just sounds like you need some better air flow when sitting in traffic.

That radiator sure looks stock to me. The fan looks big enough. One thing you could try is just adding a spacer to the fan to get it a little closer to the rad. It looks a good 3-4" away from the core?

If an OEM shroud is not available for that model, you could have one fabricated out of aluminum. I've also seen universal shroud kits that aren't that expensive and should help pull more air through.

Lastly, while I cringe when anyone considers putting an electric pusher fan on a radiator (not very efficient and will block air flow on hwy), that solution is not all that bad if you have space in front of your radiator. I have seen hundreds of cars use a pusher fan successfully despite it being a band-aid fix.