Like BigJim said, I'd stick with the stock replacement mechanical pump for a carb'd engine, it will last 10 years or so and cost under $20. The late 258 fuel pumps include a roll over valve in them.

I would add that you should carefully inspect all of the rubber hoses on the suction side of the pump all the way back to the fuel tank sender, any small crack in the hose or a loose clamp can allow the pump to pull in air instead of fuel. It may also be seeping fuel. If the rubber hoses are over 10 years old, replace them, they don't last forever! Usually, the stampings on the hose contain a date code. I'd almost bet that your rubber hoses to the tank are from '76!