Yes it's suppose to start spilling out if cap is off if you run it for a while (if you rev the engine a little, you'll see water go down and come back up when you let off).
But you need to check and make sure your sending unit isn't screwed. I would suggest once again, drive the car and if/when temp guage goes up to ~210, shut off car and listen for boiling or look for overflow or excess coolant in reservoir. If none of the above, then it is not overheating. So change your temperature sending unit.
It is NOT an odd thing for the sending units to go bad. Don't go buying a brand new radiator when it could be something as simple as a sending unit.
One more thing, when it gets to the supposed 210 degrees, when you go to shut off the car, it'll most likely idle in park at a higher rpm than usual and then when you shut it off, it MAY shut off and then kinda sputter a little (as if it wants to crank back up).
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'79 Mustang Coup
4" Cowl
Weld wheels (15x6;15x8)
Stock 5.8L
C4 w/ shift kit
1.5" MAC full length headers
Holley 750 CFM
Edelbrock intake
2.5" Off road H-pipe, 2-chamber Flowmasters
8.8" Rear end w/ 3.55 gears (Now that's more like it!!)
Front: 225/60/15 Rear: 255/60/15 Eagle GT II
14 x 4” K&N air filter (getting the Xtreme setup soon)
"Red, thou art my companion. Hasten now your quickened metamorphosis to Green that I may conquer all who dare abide there beside me. May they be left thither behind burnt black." ---Fox Body