-6an for transmission lines?

Adapters are no problem, the trans is pipe thread. AN fittings are same as JIC, used in tractor/ industrial hydraulics. A no6 JIC/AN to pipe thread is easy to get. Don't remember if they are 1/4 or 1/8 offhand

"The trick" is that AN/ JIC are 37 degree flare, the original inverted flare or male flare (brass) are 45 degree. No5 (5/16) will screw together but the flare angle is different. No6 JIC/AN is a different thread than no6 SAE (45 degree)