Personally, I would use a 5 pin automotive relay wired like this.
A wire from the starter solenoid goes to pin 87A which provides power to the fuel pump through pin 30 when the key is in the Start position.
A wire to from a lead that is hot when the key is in the Run position goes to pin 86.
When the key is released to the Run position, the relay is energized, pins 87A and 30 open, pins 87 and 30 close and power is maintained to the pump.
If you want the option run the carb "dry" you could add a toggle switch to pin 87.
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