'67 cuda slant 6 transmission cooler line fittings
Should be 5/16 SAE flare. That is NOT AN or JIC Any good parts store including No Auto Parts Available and O' Really?'s should have them, either flare nuts for tube, or "push lock" hose ends
Actually, ACE hardware might have them, but they might not have 5/16. Most household plumbing (ice machines, etc) use 1/4 or 3/8, no 5/16
These are what is called SAE which is 45* flare angle.
https://www.fastfittings.com/uploadvideo/product/30250.jpg
What you do NOT want for a factory radiator is JIC / or AN which is 37* flare angle
The original flare nuts were steel with steel tube. What you will find in the parts stores are brass nuts
One type
another