I second what Doug says, the hot tanking process doesn't remove the rust scale in the water jackets any more. It becomes loose and flakes off and plugs the radiator.
This is not necessarily true with his car. I ran my old '64 and my current '65 Darts with the same radiator and it had a crack along the bottom of the top tank. I just loosened the radiator cap and keep going.

Newer cars require a pressurized cooling system because they run hotter i.e. over the boiling point of water. They do this for emissions and engine efficiencies reason. Pressurizing the system raises the boiling point.

That you got away with this doesn't mean everything was all fine 'n' dandy. Quite the opposite, in fact.
With water-based coolant, our A-bodies require a pressurized cooling system just as much as newer cars do. Thermostat temperatures haven't gone up much -- some cars still use 195° thermostats, and 205° is about the highest you'll find.

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Too many people who were born on third base actually believe they've hit a triple.

Actually it is true. Even though it is an older model car, it is still a pressurized system and the same principles still apply. Lowering the boiling point by 45 degrees definitely makes a difference.

Sounds like your cars didn't have an overheating problem, just a leaky radiator problem so you got away with it.

It's important to keep in mind that there's a lot going on in the cooling system that we can't detect because it's not happening up near the top/front of the system (radiator & hoses). Local temperature inside the engine, right at the metal, can and will be much higher than the boiling point of unpressurised water or any water/regular coolant mix. Local boiling can and will occur, even if we can't see it at the radiator. This can cause ping and other problems. Detailed discussion can be found in the threads discussing waterless coolant.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

When you are broke, you do what you can. When you're not, fix the problem.

That and the "$200" car theory, will usually get you thru.

Quick update...
After messing with it and trying to plug the hole, I figured out the radiator was just super clogged with rust n all kinds of debri from before the motor was rebuilt.
Had it cleaned out really well, the hole soldered and it's back to being perfect.
Thank you everyone for your help!