Temperature Sending Unit Size

Stock hole is 3/8" NPT as I recall. AM tends to provide a SBC adapter instead of a Ford adapter.

Hissin is right on. The adapter that comes with the autometer mechanical gauge is a 1/2" npt. They sell a 3/8" npt adapter that takes the place of the 1/2" one. Its only a few bucks and works great.

Not sure if this would be of interest to you....but...
I have a mechanical H2O temp guage hooked up on my car. I bought a new water outlet neck from Auto Zone that includes a threaded plug port. The Autometer adapter that is included with the guage fits perfectly. I just substituted the new water neck (which is only $10 or so) for the stock one. Threaded the AM brass fitting into the top of the neck, ran the pickup from the guage to the neck, threaded right in. Keeps the stock unit working and looks great.
Just an idea for anyone looking to hook up a water temp guage.
Ken
This is the new neck with the autometer fitting installed.