Good advice from Capri306.
Make sure you haven't overfilled with coolant first. Your topic says the car is actually overheating. Is it or is it just overflowing from the reservoir? If it is just overflowing, then you have too much coolant. If it is overflowing AND overheating, then, most likely, the water is not being cooled efficiently enough. Thus as stated b4, I would have the radiator check out first and then thermostat. When was the thermo replaced? Last few years? Maybe time for replacement.
Lastly, I believe the water flows out of the radiator through the bottom right hose (if you are facing the firewall), into the engine and out from the engine, through the top left hose (while facing firewall) into the radiator.
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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