Struggling under load

Just more ideas:

Since it runs well when you accelerate it might not be the fuel related. Making changes there does not help. It may be the ignition. Are you running points or electronic?

It may be transmission, but not sure if it is possible. Not sure a torque converter can get tight like a clutch.

If it runs well when not stopped, that is a huge clue. Perhaps someone will have an idea about that. I assume the kick-down linkage adjustments are correct, not to interfere with curb idle setting. Does kick-down work correctly?

It is possible to have a bad engine to firewall ground that permits an engine to run when accelerating, and die when coasting, because it grounds via drive train under load.