Keith,

adding a tap to the header or pipe doesn't necessarily require it to be backed by a nut. If you drill the pipe or header carefully and with the right bit, you can then "tap" the aluminum with a threaded nipple. If you have a gasket type washer at the base of the nipple, when you screw it down snug, it will stay in place just fine. This allows one to put the nipple at the widest part of the diverging cone of the pipe which is otherwise inaccessible from the interior.

David