If your intercooler piping is thin aluminum, it takes a lot of skill plus the right equipment (TIG) to weld a bung on. Forget about tapping 3/8 NPT into thin aluminum tubing. You cannot reduce 3/8 NPT to 1/8 NPT, you can expand it but you don't really want to do that as the mass and leverage of the contraption from vibration will break the tubing in short order.