Tempeture Sending Unit?

[Mr Handy]

I hit the parts store for a new temp sending unit last night and it is wrong and reads hot on the stock gauge in my 70.
Now that I have 2 of these sending units I recall that I had an old one in my old engine that worked fine. I pulled it and tested resistance in hot water and room temp and the resistance grade is way different. Much slower on the old/too fast in the new.

Does anyone still sell the correct temp sending unit for our trucks? I would like to have a backup if the old but working one dies. I was surprised I could not find this info searching. I would think everyone with a small block would have walked this road... maybe I'm crazy.