Dart wont stay running

When they idle poor, it is usually the other way around. They idle fine in P or N, but slow way down and shake in D. That is usually a lean condition. Since yours acts opposite, maybe you have a rich idle. Try adjusting the idle mixture. I am guessing your 2 bbl is not original since I thought the 2 bbl manifold (Super Six) didn't start until ~1976.