I had a similar problem with the original 216 I had in my '40. I wanted to use a modern temp gauge and no matter how had I tried, I could not get a unit to fit. One major problem I found is when I found the proper 3/8"-to-1/2" NPT adapter, the new temp probe would not fit down into the 3/8" part of the adapter. The bulb was too big! One was too short and it bottomed out against the head! One I just couldn't get to stop leaking! I never had so much frustration and over a gauge installation!
I solved my problem by going with an electric gauge instead of an mechanical one. I love mechanical gauges as you know when they don't work. The electric ones, you just never know.
With a 6 volt system and 12 volt gauge, you use a resister to step up the voltage. You can buy those at Chevys of the 40's.
Eventually I went to 12 volt system and a more modern block.
RapRap
1940 ChoppedChevyCoupe
