I tapped right at the thick part on the collector flange. I pulled the inner plastic fender well out to get to it. I think something was in the way to get to the rear cylinder hole. The cast iron is thick here. Along the top of the manifold, the metal isn't the thickest. For 1/8th NPT you use a "R" letter drill and a 1/8NPT tap if the fitting takes a taper thread. Grease the drill bit to catch the chips. The hole is where the silver plug is. I put a different manifold on and am selling this one so I plugged the tapped hole.
View attachment 109076 Grease the tap to catch most of the chips. Before installing the probe, start the engine and it will blow most chips out. Any left will harmlessly go down the exhaust