I'm not sure how critical the spacing is. Obviously it has to be on top of the liner; and I chose to put it in the slight depression just outside of where the copper sheet from the other side is folded over. But I wasn't particularly careful about making sure it was right against the edge of the sheet. Here's a closeup
The "tack weld" is just soft solder. (Kester 63/37 eutectic rosin core, just because that's what I had on hand.) My assumption is that it will deform and flow under the clamping force from the head, so I didn't worry about particularly making it even. In fact you can see in the photo, it was a fairly lousy soldering job. The idea was just to ensure the wire staying in place while the head was installed. (I did go back after the photo was taken to cover up the ends, just wanted to show how they were overlapped slightly.)
FWIW, Ken Gillanders (BFE) suggested using super glue instead of solder. Makes sense to me, but I stuck with solder since it worked so well for me before.
Also FWIW, this time around I used "Tarn-X" to clean up the copper surface before soldering. Lots less work than shining it up with Scotch-Brite or steel wool, and seemed to work fine.