It is 3/4 NPT. Yes, it appears to be too small, but you need to grind off part of the tap to make it work just right. The NPT is tapered. Get the starter threads ground down about .250 and you get to the better threads and be sure you tap all the way up in the housing, but do not force it at the bottom out area. The housing will crack if you are not careful.
Once you have the tapped hole, put thread sealer and Teflon tape on the threads of the fitting, insert and be careful once again and do not bottom out into an area that does not have the threads or you will crack the housing. I have tapped about 25 of these for various installs and other people.
http://www.fastfieros.com/projects has many pics and at least 3 installs have the fittings for heater core return support.