Hello  Although I 'am not sure but, have noticed on large (3/8") temp senders  stamped with numbers.  I suspect  they are numbers of water temps at max temps.

On my 56 292 I added a mechanical Temp Gauge by having Manifold drilled and tapped at a local Machine Shop. Casting is very thin in this Area but I have done 2 of these and not had a leakage Problem. I also added a 6 Blade Fan and Napa Thermostat THM-6. 170 degree Stat with larger opening than most standard Stats. The Mechanical Gauge seems to read 165-170
most of the Time. The original Dash Gauge reads around 3/4 towards Hot. Obviously there is a difference between sender located at back of Engine (stock) and having One directly behind Thermostat Housing. The only other thing I would like to change is the Water Pump Pulley to a smaller Size when I find One. I also removed the Air Deflector on Top of Rad Support. 2 Schools of thought regarding that Deflector. Some say it was there to retain heat in Engine Compartment in Winter. Others think it is there to force Air through Radiator. I don't drive it in Winter so I took it off.

  

READ HERE - https://www.ctci.org/1955-56-57-thunderbird-overheating/

AND HERE - https://www.fordbarn.com/forum/showthread.php?t=316470

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Ross, it sounds like you've done your homework, and you've hit on most of the things that have been addressed regarding what keeps our engines cool (or not). Unfortunately, what works for one might not work for all. 

Before taking my bare block to a machine shop to be cleaned and magged, I removed all of the rust in the water jackets between cylinders. compared to images posted on CTCI Gil's Garage, mine looked pretty good. However, after the blocked checked out okay for cracks, I took it to a self-serve car wash and pressure washed it by inserting the pressure washer wand into the water jackets at the expansion/freeze plug holes and at the front of the block. I flushed out a lot of rust and rust-colored casting sand. 
Engine condition and build quality can also make the difference between a cool engine and one that overheats, particularly if the head gaskets weren't correctly installed.   Hopefully you'll get yours sorted out; if so, let us know what was required.

I've tried 3.  180 degree with large opening, 160 with regular opening, 160 (MR Gasket high flow).   The only difference between 180 and 160, the 160 was cooler UNTIL the engine and engine compartment reached full temp.   Then the 160 didn't make any difference. The 160 with regular/smaller opening seemed to run the hottest

I have a 170 with large opening, I will try that eventually.   I don't think the thermostat is the issue.

It's amazing how much contradictory information is "out there".  I've read:
(Use regular gas, Use premium gas) 
(More timing is better, Less timing is better)
(Use an auxiliary pusher fan, an auxiliary pusher fan could block air flow) 
(Brass/copper radiator is better, Aluminum radiator is better)
(High flow water pump is better, High flow water pump could pump too fast and radiator doesn't have time to cool the coolant)

What thermostat are you running?  I switched to the NAPA #6, 170 degree thermostat a few years ago and it made a HUGE difference in keeping it cool.

You said you have a new gauge and sender, have you checked the wiring? High resistance is very common on these old cars and depending how the sensor reads or where the resistance is can cause low or higher than expected readings
Seth

Thank you. I did not realize moving the sender will give a higher reading.

This was today (97 degrees), the car never boiled over.  When I got home I checked the temps around the engine around thermostat 215, back of heads 220.  Radiator top tank 190, bottom tank 160

Driving around town (around town the gauge is about middle of scale) and a few minutes on the highway with the AC on






This was at highway speed (avg 75)  It's unnerving to ride around on a hot day with the gauge pegged:

The Ford Y-Block temperature sending location at the back of the engine typically reads 15°-20°F lower than what is seen at the thermostat.  Moving the sending unit to the front of the engine will have the factory temp gauge reading a much warmer temperature.  Relocating the sending unit is recommended when adding a non-stock temperature gauge, but the factory gauge will simply show a much hotter temperature when in reality nothing changed.  Converting these engines to an aftermarket fuel injection setup will require that the temperature reading come from the front of the engine so it can have access to the warmer engine temperature reading.

Lorena, Texas (South of Waco)

The sender location isn’t ideal, but those old gauges weren’t all that accurate to begin with. Since they’re the King Seeley’s you can’t just add a resistor. There is some adjustment, but it’s almost impossibly to do unless they’re on a bench, hooked to a battery, and you can make small changes and wait.

Blacktie has the best solution. A 2 or 3 gauge panel. Water, oil, and volts if you’d like.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ