I deleted my last post as it contained incorrect info. As HRL said, it is a special bushing. My ‘52 with a ‘59 235 has a bushing. It looks to me like a 1/2” NPT to whatever the temperature probe is. Here is one from from
Classic Parts that may work. It’s also possible that you have the wrong sending unit.
Here is a couple of pictures of mine, but keep in mind that mine is a direct reading gauge.
