76 Duster Overheating at idle

Matthew Sermini said
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.

That's probably not vapor lock. Vapor lock is what happens in the fuel lines. The carburetors runs off the fuel in the bowls. The only heat related restart issue that can occur is when vapor in the line expands with enough pressure to force open the bowl's inlet valve. If there is liquid fuel above that expanding vapor it can cause flooding (bowl overflow).

It could be the choke or cold idle settings or too advanced initial timing. So based on the information so far there's not enough to conclude the initial is too advanced or not sufficiently advanced. Although this is probably not the cause of what you observing you should know that:
A factory 340 will be fine with something like 12 - 14* BTDC at 650 rpm. A hot rod will want a bit more initial.
A stock factory distributor will be set up for something closer to 5* BTC @ 750 rpm for emissions purposes to put more heat into the cylinder walls. All 340s were produced with a Clean Air Package so will have distributors with the long fast advance off of idle.

Matthew Sermini said
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.

Based on the difference between moving and not moving, that points to a difference in air flow between moving and not moving. Fan shroud and maybe a front hood seal if not present would be the first things to try.