Oldray, I don't do much AC, I'm a windows down kind of guy. That said, I doubt there is enough material in an AC line to cut threads into, the lines I've seen are way too thin.
As far as welding, Stainless is usually pretty nice to weld to. If the AC lines are steel with an aluminium coating, you will have to remove the coating to weld to the stainless fitting. I would suspect a gas weld or a tig weld would be the best method, because its too easy to burn through thin tubing with a mig. A compression fitting may work, but I would really want to watch for a freon leak using a compression fitting.

They have been making custom length AC hoses for years, there has to be a simple way to accomplish this, might be worth consulting a home AC installer. They do custom hoses all the time. Gene