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Non-ethanol fuel may have slightly less tendency to vapor lock. "Regular" grade fuel may be a little better at resisting vapor lock than "Premium". If you add an electric fuel pump, the closer to the tank you can get it, the better. Wrapping the first 5 feet of the exhaust pipe with insulating header wrap may help reduce under hood heat. Wrapping the portion of the tailpipe that is near the gas tank may also help. Be sure your manifold heat valve is working as it should, and that ignition timing is correct and the spark advance mechanisms are working as they should. All heat shields should be in place. It's important to keep the radiator core clean and free of bugs and dust to maximize air flow. Any badges, license plate, accessory lighting, or any other items that could obstruct air flow to the radiator should be removed or relocated. Today's "gasoline" is designed for non-vented, pressurized fuel systems and to suit EPA mandates, and it's less than ideal for low pressure carburetor fuel systems with underhood fuel pumps.

I've seen a picture somewhere that shows a small sheet metal shield over the manifold heat valve thermostat spring on a 120 eight. My car is missing this piece. The manifold heat valve seems to work pretty well on my car, at least when it is hot and sitting with the engine idling, but I wonder if the lack of the small shield over the thermostatic spring might allow the heat valve to partially close when running down the road with a lot of under hood airflow. If it does, it could contribute to vapor lock when idling or after a brief shut down in hot weather by causing the manifold to run hotter than normal.