troubleshooting 73 dart slant six...

It looks like you're not too far away from me. If I knew more, I'd offer to help, but the best I could probably do is sit with you while it dies.

Back when my 73 Duster was a slant six, I never had much luck with rebuilt carbs, but you can get lucky once in a while. Since it happens with both carbs, could it be something with 3rd gear in the transmission? Have you tried keeping it in 2nd after it heats up and seeing if it dies out?

You can also try running it with the vacuum advance plugged to see if that is contributing to the problem.