My 250 (from a '74 Nova) has the original "idiot light" switch under the thermostat. When I mounted the gauge, I put the sender in the left side of the head.
The truck, '65 C10, has green & red lights on the dash, presumably for cold & hot, respectively. I've never seen the green light come on. I might have the wrong temp. switch?

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If you can't fix it with a hammer and screwdriver, you need a bigger hammer.
1965 Chevy C10