engine and drivetrain tii oil filter housing

[JHG762002]

Does anyone know what size bolt I should use to plug the outlet on the oil filter housing that fed the kugelfischer. I managed to plug the return port on the block, but neglected to plug the supply. I have carbs on the engine, so no need for the outlet. I hope its something I can just get at the hardware store. I need to get this car on the road, and that mobil one was expensive.

[finkbuilt]

You could always plug it with an oil temp sender, then you'd just need to find a spot for the gauge.