Stalling in heat help

[Mgicm]

People in this forum have helped tremendously on some other issues Ive had and hoping this one will be met with the same help. At my wits end. let me explain.

Have a 73 GMC step van.. 3/4 ton underneath. 350 sbc with edelbrock carb. HEI. So last year when it was over 90 degrees out, i would get some backfire occasionally in carb..like pops. Changed over to HEI and new carb. Seemed fine. Then we would take a run on highway say 30 miles and when we got off highway to a stoplight, it would stall. Temp gauge in middle so didnt seem to be overheating. Check radiator and overflow..all good. Ended up changing distributor 3 months ago as mechanic said it was coming up as bad on his tests.

Fast forward to now the middle of summer. Today was hot.. 92 degrees. Drove to a destination about 30 miles away. Drove perfect..mostly highway but some local miles/stoplights. Got to destination and parked the van for 2 hours.

Tried to start and it stalled out. Got started, drove back on the 30 mile route and there was tons of traffic. Stalled out about 20 mins into our drive back. Could not restart it. Opened the hump inside stepvan to let heat out. Started after waiting about 15 - 20 mins. Got some miles in on open highway for 15 mins. Then back to traffic but now i have hump open inside to let heat out. Stalled again but was able to start it quicker this time. Bumper to bumper traffic all the way home but kept it running. Get home , shut it off and it is running/bucking/dieseling/popping for a good 15-20 seconds after I shut key.

So my guesses vary here as to where the issue is. Gas boiling in carb/vapor lock? Timing wrong/running too hot? Engine compartment too small for such heat of motor? Fan not working as expected? Distributor bad again? Gas line too close to motor and boiling in traffic/heat?

Anyone have thoughts or opinions? Bringing to mechanic but want to have some sort of education behind me . Sure appreciate!