Most factory coolers run solid steel tubing lines, but if you want to add a short piece of flex line between the trans and the body it will avoid getting stress cracks at the flare fittings. Just be sure the flex line is rated for carrying hot oil somewhere in the 250 degree range, as trans fluid can get pretty hot on a hard pull.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!