76 Duster Overheating at idle

I have some questions I was hoping I could get help with. I have a 1976 Plymouth duster. Original slant six car now with a 340/904. I have a 22inch aluminum radiator in the car (only one that fits with original mounting points) but it seems to get hot a little too quickly at idle. I took the car out last weekend and it was getting pretty hot within 5-10 minutes of idling around at a show. The car doesn’t overheat when going down the road but I’m looking for solutions for when I’m in heavy traffic or at a car show. I don’t have a fan shroud on, and everything is within a couple years old. Car also doesn’t like to start if I go into a store for like 20 minutes but as soon as I get back on the road any vapor lock issues clear up. Newer thermostat and new fuel pump in the car. Any tips or input would be great.

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