I think the original line was steel. Copper would work better as it won't be as likely to rust through when exposed to coolant. The specialty fittings that seal the coolant aren't totally necessary, but by the time enough fittings are purchased to bypass them, that kit price starts looking pretty good.
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!