Manifold EGR Jets

I have a '79 casting date spread bore manifold that I got in a parts deal a while back, just getting around to using it now.
It already had the EGR jets removed from the plenum floor, leaving just the open threaded holes, so I tried to plug them- tried pipe thread, fine thread, coarse thread, nothing seems to fit. Was this a special thread, and does anybody make plugs for this? Almost ready to just run pipe taps into them, but thought I'd check here first.