‘71 Dart with 340 motor stalling problem

Sounds like you have a common 71 fuel vent problem. Drive it with the cap off . Make sure the vent line coming to the engine compartment on the passenger side is open. 71's have a none vented cap and a sealed vent system that ends on the breather cap on the passenger valve cover. I have seen these lines plugged. and or crimped shut. Or rotted through at the vent tube and clogged with rust.