correct filler neck for 70 swinger 340?

All A-body tanks are vented one way or another. The tanks up through '69 everywhere (plus '70 except California) have a simple single vent to atmosphere (it has a high loop in the vent line in the trunk to prevent liquid fuel drooling out onto the ground). Vent outlet between tailpipe and fuel tank (can be fun when the exhaust guy's not paying close attention to where he's pointing his torch). Single vent at top front centre is '72-up tank. See here for info including the 4-vent '70 California + '71 50-state/Canada tank. If your car originally had California emissions, it would've had that 4-vented tank (and a cap that is very difficult to find). If I were working on a car like yours, trying to optimize for function (rather than doing a 100-point accurate-to-1970 concours restoration) I would use the '72-up style tank (the kind you have now) and put a charcoal can in as described at the link. I would also use a '72-up filler neck to make life easy on myself when looking for a gas cap.

People use that phrase "vapor lock" to refer to a lot of different symptoms. What exactly are you experiencing?