I Stitched my Guillemot hull up with 18 gauge copper at about 3 inch spaces.
A few wires broke on me but, not enough to worry about. 18 gauge is real easy to twist too. I started out with lengths of 4 inches but, they got shorter and shorter as I got a feel for it. By the end, I was using 2 inch scraps with good results.
About the steel wire. The only concern I would have would be rust. Sure, there isn't supposed to be any water in there but, it wouldn't take much to make a stain under your 'glass and varnish.
Hope this helps.