Suspect Temp Gauge of not working but need to test

I don't remember the ohms for that but an easy way to test is turn the key on and ground out sending unit cable to engine block. If the gauge moves to hot then it may be the sending unit even though it was replaced. On those the gauge ran above the middle on the line or does it go all the way to H? If the engine has been modified your radiator may not be large enough for one thing eve though it was replaced. Also you can put a water pump off of an a/c car on it as they had different impellers. And use the radiator cool down stuff it works. One other thing is to check to see if lower hose is sucking together when it's revved because some of those had a reinforcing wire in the hose to stop that kind of stuff or get a hose that isn't fitted as those have that.

Your gauge is bad or make sure the sending unit has blinking light with a test light if that has a blinking light then i'd say it's a gauge but I don't remember if those have a circuit board or not. If it is direct wired try switching your wires in the back as they may be backwards. It is definitely not the sending unit. Are these factory gauges or a aftermarket, if they are then i'd say wiring is backwards. I dont' remember and I've been doing this for 50 yrs if there are wires going to gauge try switching them and see if it works then. I don't remember if it had a flimsy circuit thing or not.