Welcome Chevrolet Canada
Agree with John that your truck qualifies as "Neat"! Also, you are not the only member with a C15A. There has actually been some recent discussion about some difficult to find steering tie rod ends for the C15A and looking for possible sources.
On your temperature gauge question, need to know if the gauge with sensing bulb (from your picture) you are looking to install is the one meant for the original 216 engine?
If yes, it is designed to fit the earlier 216 head probe opening. You will need an adapter fitting (usually brass) to fit that gauge bulb into the larger 261 temperature probe hole.
That adapter fitting is available on this side of the pond from a number of suppliers. Below are links to 3 of those sources:
https://www.napaonline.com/en/p/BK_7011798https://www.classicparts.com/1941-54-Temperature-Sender-Reducer-Bushing/productinfo/24%2D811/#.UP9MwXy9KSN (must copy and paste this link)
https://store.fillingstation.com/detail/FS-1001/Chevrolet_19301962_TEMP_GAUGE_FITTING_IN_HEAD.htmlNot sure if you have sources in the Netherlands for fittings with these specifications???
From what I have been able to learn, these fittings (normally brass) have an internal thread size of 3/8" NPT (National Pipe Thread) with a beveled seat that your existing probe/collar nut would thread down into, and the external thread size is 1/2" NPT (National Pipe Thread) which is threaded into the temp probe hole in the drivers side of the head towards the rear. Picture attached below to show older probe installed in head with adapter.
Hope this information is helpful.
Once again, Welcome!
