bkdraft, there are different ways to do this and depending on your engine would depend on the thickness of the plate. On my OS 91FX I tapped dead center, but first I made a think layer of JB weld to increase the thickness of the wall so that the nipple did not protrude through and stick out on the inside. I then drilled and tapped through both walls. By me saying this is not to undermine your intelligence but make sure you remove the backplate before any drilling, the instructions state this as well.

Blackie