How do I stop vapor lock?

Venting varies significantly by model and year from 69~70's. If you have a vent on your filler neck and at the sender, one or the other should probably be open. My '72 has a vent line from the tank up to the engine compartment that's attached to the top of the tank.

So long as it's filtered, and possibly even a roll-over valve, having one of them vented is likely a good idea so long as it vents somewhere other than into the trunk! It should go outside to the atmosphere. It should also be above the top of the fuel tank..