Does anyone know if 3/8 fuel line is OD or ID? Also what is the wall thickness? I have a supplier of stainless steel tubing and he has no idea. All I told him was the pressure requirements are very low, never more than 20psi. Hope that exceeded what is needed.

It will depend on the material. By definition, the SS tube should be 3/8 OD. But if you have 3/8 rubber hose it will be 3/8 ID. The wall thickness of the SS tube is probably insignificant with regard to fuel flow.

Just make sure that the supplier can supply the proper fittings for the hose.