I haven't done what your wanting to do yet so keep that in mind. My understanding of the oil routing is the oil from the pump goes through the block through one passage, the smooth smaller one your wanting to drill and tap and returns to the motor through the big hole, correct? If so then make the oil feed hole a little bigger than it is(#8 AN or bigger ) and use full flow fittings so you don't restrict the oil flow
Make sure and put the threaded plug back in to route the oil correctly