Update:
Using a couple of online parts store application searches, I found out what temperature switch w/light is supposed to fit a 1969 Chevy truck with a 350 engine. For example, on Autozone's website, searching for this gives a TU51 part number. From my bucket loads of online research, I recognize that as a Wells part number. So it is a Wells switch put in a Duralast box for AutoZone.
Now, over to the Wells website to find out the specs. After looking around for a while, I figure out that the Sales Drawings are what gives the specs. It switches at 251 +/- 10 degrees.
https://cdn.gotech.parts/sales-drawings/TU51A.pdf
or
https://web.archive.org/web/20230114...ings/TU51A.pdf
I wrote to Wells for technical support and asked this:
I have a 1969 Chevrolet C10 Pickup with a 350 engine, and it calls for a Wells TU51 temperature switch. I’ve been looking at the specs on your website and that switches my warning light on at 251 degrees. I had my engine rebuilt, and it has new heads installed that work with modern gasoline. The thing is the new head has a 3/8 NPT hole for the temperature switch, where the old head had a 1/2 NPT hole and accepted a TU51. I had a look at fitment for later model trucks and found that the temperature they switch at is 263 degrees in 1970 and above, and in 1981 when they switched from 1/2 NPT to 3/8 NPT they continue to be 263 degrees, so I guess that is 12 degrees higher.
What I’m wondering is do you have a switch for about 251 degrees with 3/8 NPT?
They wrote back:
David, p/n TU78 closes between 230 & 250* and has 3/8-18 thread, this was used on '82 to '89 Chev & GMC trucks. - Glenn
So, I think I'll put a TU78 in the passenger side head and connect to the TEMP light.
https://cdn.gotech.parts/sales-drawings/TU78.pdf
or
https://web.archive.org/web/20230114...wings/TU78.pdf