I have a 73 cougar 351 2V the temp guage goes to hot quickly and stays there,

If the sending unit was replaced, maybe something wrong with wiring circuit? Maybe air trapped, somewhere? Did you take the temp reading at several places, all show the same?

If the engine gets too hot, it will pop the radiator cap and the coolant reservoir, where applicable, will fill up in a hurry.

My wiring info for your model year, isn't very good.

Temp readings are the same, the engine is not getting hot enough for the rad to boil over. The fluid was just change for the rad replacement so I don't think it is an air lock. Could it be that the gauge is not reading correclty for some reason. THe flow seems to be good but changing the rad did not make any differece as far as the gauge reading goes.
... Ted

What about your sending unit for the temp gage? Is it one wire? Usually the sending unit controls resistance to ground. If you unplug sending unit, then turn the key on, any reaction from gage? Is there any voltage on that wire, with the key on, use digital multimeter? Like I said, I doubt I can find wiring info on that gage?

Usually with sending unit unplugged, the gage shows cold. With the one wire , quickly grounded for a second, I don't like to peg an analog gage, the gage reads hot. I just can't be sure your gage works exactly that way?

How many wires coming off temp gage?

The sending unit is one wire. I have replaced the connector and 4" of the wire. I am not sue how many wires comming off the gauge, would have to pull the dash apart to get at the instrument cluster. I replaced the gauge a couple of years ago I think there were 2 wires but not sure. Can't find the old gauge to check it. Anyway that did not solve the problem.
If I unplug the sending it reads cold no reaction, with the key on or off. I do not have a multimeter.
... Ted

Take out the thermostat out. Hook everything back-up. Drive it for a bit. Does it still get hot? If so, Maybe combustion gases in the coolant?