75 Dart Swinger Transmission to Radiator lines wont come off

ALWAYS use two wrenches

Yea, and preferably line wrenches.:D

If need be the ends of the line can be cut with a tubing cutter and the fittings in the radiator can be replaced with compression union type fittings.
These are what I used when I swapped the lines and fittings from 5/16 to 3/8.
Been about a year of daily driving with zero problems with them.

imagesVYDRC69H.jpg