Two things come to mind. Many years ago, when getting my 70 Charger running, it wasn't getting fuel to the carb. I replaced the pump and even the rod that activates it off the cam. Ran fine with an electric pump plumbed from a gas can with rubber hose, but . . . Anyway, even though the lines looked ok and were clear, there was a small pin hole or two in the main line not allowing the pump to form any suction. Also, many hands have had their way on these cars over the years, and not all of them competent. When I got my 73 Dart home a few years back, it wouldn't stay running. The previous owner had dropped the tank to seal a leak and when he strong armed it back into position, he kinked the rubber line. Think and look outside the box for other peoples "workmanship" issues.
Btw, my Dart had so many ineffective hacks performed by the PO that I contemplated naming it Alice: because a butcher had his way with her.
