Dying Dart after being warmed up.

The 71 service manual is a free download here somewhere. From that you can determine does your car has ported vacuum switches and other emissions features that can cause problems after the engine warms up. Onlt place to attach a booster is the main manifold vacuum port. It can be any type from a single port to a ( easily broken ) tree of ports on 1 fitting.
Again the factory service manual contains most of the answers you need.