The first person you need to talk to is your doctor because there may be issues with being close to the arc. MIG leads over 20' long do not work well at all and this is with much larger wire than what you will be using for body work so I don't think you would have much luck with it even if you could find one that long that fits your machine. Maybe a spool gun designed for Aluminum might solve your problem, I have never had any reason to try to use one with steel wire but maybe some of the other guys have. This would eliminate the problems with trying to force that tiny wire through a long cable. Have you thought of torch welding? Some body workers actually prefer to torch weld and in fact I did much of the welding on my 65 with a torch (Q-panels, radiator support patch, etc) and just recently I welded a lower door skin on a 68 with my Victor torch and a no.000 welding tip. This is the way it was done for many years and it worked then, still does today.