Boyd welding has a slick little adapter plate that allows you to use a factory Ford vent on a custom tank. You could use the Boyd vent plate on a custom expansion tank. Build a one gallon expansion tank and plumb it to the existing vent on your tank and then use the Ford vent on the expansion tank. Or if your existing tank has a vapor dome in it then you could use the Ford vent right in the vapor dome.

https://fueltankparts.com/collectio...ucts/ford-oem-vent-weld-in-bung-aluminum