68crazyhorse 210 is slightly hot. You want to keep you temp 160*-190*. What thermostat are you using any mods latley?? Any work done to the heads? How is you timing and the running condition of your stang??

my thermostat is at 180 degrees the head are rebuilt not too long ago. my timing is set at 25 degrees. it's carburated with a high raise elderblock intake. maybe this can help

Is the water pump flowing the fluids thru the system?? Disconect the top radiator hose, disable the ignition and crank it a few time see if water pump is pushing. Also there is sometimes when the thermo get stuck, after engine has warm up to normal temp check the pressure by hand on the same hose we discusssed.The valve should release or open when it meets the thermostat temp- you are using.

The old downflow radiators don't work as well as the later crossflow ones do. You want the proper waterpump, as serpentine belt 5.0s rotate their waterpumps backward. The temp gauge sender is different between the early 'Stangs and the later fox 5.0's, though a later sender made the gauge read lower (cooler) in my '68. Other than that,, make sure your radiator is good, and that the thermostat and head gaskets are installed properly.
But first, you should hook up a mechanical temp gauge and verify the temp.