correct filler neck for 70 swinger 340?

But remember, when you floor the accelerator, you're also letting a lot more air into the engine.

It does sound heat-related; I'd look at a thicker carb base insulator and a heat shield. Also note the MP electronic ignition is not a good pick (without tweaks) for street driving, and reliability on the MP modules has been "poor and getting worse" for quite a few years now.

Non-vented cap would be correct for a '70 non-California car, but no harm or trouble comes from using a pressure-vacuum cap on a car that doesn't need it (what you can't do is the other way round, unvented cap on a car that needs a pressure-vacuum cap).