Slant Temperature

I have the factory gauge on the dash showing way way hot. Then I have an aftermarket gauge on the passenger side plug of the block.... which would you trust and I think I have a cooling problem now. It’s only 85 today and the needle is pegged to the right on the dash and the manual gauge reads 195.

66 Val 170 slant six

Who makes a high flow pump and where do I get it?

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What size is the plug in the block. I'm wanting to put my after market temperature gauge there