On the top of the block near the distributor are two oil gallery holes. One is normally used for the oil pressure gauge sender or idiot light sender. The other is normally plugged. Both are 1/8NPT.
The gallery hole pointing straight up will be redrilled and tapped for 3/8NPT similar to the holes at the pan rail. Another one time use jig could be used to align the drill (using the distributor hold down hole for attachment), but in the case pictured there two of us so we were just very careful to align it perpendicular using a steel "L" ruler (not pictured) by sight.
The bottom (angled) hole is unused both before and after conversion.