‘71 Dart with 340 motor stalling problem

Is this a hydraulic lifter engine?

Then I would check the plugs,when she stalls, I bet they are overheating, indicating a very lean cruise AFR.
But
During the warm-up period,you say she runs super strong, so not lean at that time.
So then, have you tried removing the gas cap at the time of the stall checking for vacuum there.

It really sounds like too-tight valve lash when hot.This can happen with hydros too, with too-long pushrods.