I wouldn't feel well using anything self-tapping in such thin metal. You should definately have some kind of plate on the bottom to spread the load out...
ARBs run at low PSI, I don't see why a heavy duty line is needed unless you are worried about them getting cut or smashed. As long as you run them safely out of the way, secure them well and leave slack for axle droop I don't see a problem. I am not going to shell out $150 for some over price SS...