Here is what I decided on:
I used a steel adapter fitting which, in hindsight, was a mistake. Aluminum head, aluminum fitting - why use a steel adapter and introduce problems with dissimilar metals? Just dumb on my behalf. So I rummaged through my fittings collection to find an aluminum one.
As you can see I had to grind a little off the top of the t-stat housing.
This is for a mechanical gauge which is what you want, IMO.