Air will get into the system every time I check the coolant level. I check all the fluids each time I drive the car so I have to take off the radiator cap to check the level. That air would get into the radiator I assume. There's no other way since there is no overflow bottle and the system is not sealed. No, I don't have a stash of old IAT coolant. I would just go on-line to Amazon to make my purchase. Sealed vs. non-sealed system seems to make a difference with Dexcool. The 1966 Mustang just has a cap and an overflow tube to the ground.gelling will not happen unless you deliberately find a very old IAT formula and introduce air into the system.
original green is a garbage low silicate IAT, do not use it in anything. if you’re afraid of a modern coolant attacking solder, use G05, which happens to reboxed in europe as Glysantin Classic antifreeze.