How do I stop vapor lock?

The reason a REAR mount electric pump can help over OEM is that the feedline is not under a vacuum. The pressure in the feedline with the pump in the rear raises the boiling point of the fuel. There of course, are limits.

"So far as the cap" remember that NO FACTORY CAP provides the vent function. In 69/ earlier cars, the vent was near the top of the fill tube.

In 70 cars with evap, and later, the vent was the 1/4 line to the front, and either that must be kept open, or else something else to replace it as the vent

The evap equipped cars ALL had what are called "pressure vacuum" caps, which allow "some" pressure to build, as well as "some" vacuum to occur, before "relieving" thepressure.

These are a PITA because changes were made over the years. There's a couple of threads on these caps

Here

http://www.forabodiesonly.com/mopar/showthread.php?p=1970887599