I use Dorman #555-115 3/8" mini freeze plugs. Dorman calls them cup expansion plugs. Press one into the passage with a .090" hole and go from there. .090" is usually too small and allows a flair up. But is a good starting point. It usually ends up close to .100". When I am lazy I just put grease on the next size drill bit and drill it in place if it needs opened up. Of course the better way is to screw a sheet metal screw into the restriction and pull it out with a claw hammer.

Be sure to check the shift at the temp that you usually run because the fluid temp can affect the amount of flair or overlap. It is hard to get perfect through a variety of temps. You may have to make a choice towards a slight flair of slight overlap.