It comes with two different adapters. I have mine but I dont have anything to measure the threads for you.
Look it up on the autometer site, I think tehy tell you what sizes it comes with. I think I used the bigger one and tapped the water neck for it.
Also if I were you Id put the autometer on in the stock location since its more accurate, the tap the neck and rethread the stock sender. Alot of people say that the water neck will crack, but if you rethread the stock sender then you will have no problems.
I did this on my old mustang but i cant remember what size it was the autometer site want very helpful. I was always told by local speed shops around here never move the stock one because you computer reads the temp and if its reading a low temp the computer is going to dump more fuel.
Eric's got the right idea. The stock temperature guage sender only is for the gauge. The ECT on the other hand, is important for trim and should not be tossed somewhere weird.
I got a stat housing from the parts store (10 bucks) that came with the tapped bung. The appropriate adapter (3/8" IIRC) screwed right in. This was for a mech gauge - the electrical guages use a smaller (1/8" IIRC) fitting.