So, a little feedback from my experience with redoing all the hard lines in my '73 Z, except for the fuel vent line. I went with Classic Tube's stainless steel version of their pre bent lines rather than OEM steel. I had them send me an enlarged fuel return line (5/16th) rather than the stock size. I took a fine scotch Brite pad to them to shine them up and the end result to me looks awesome. They are just the look I was going for against the blue paint, which I had just repainted in the engine bay. For each connection, I tightened and loosened about 6 times each as recommended by Classic Tube, and each connection sealed nicely. Also, threads are 10mm, but the flare nut is a 7/16". I bought, just for the occasion, a nice Snap On flare nut wrench to do the job so I didn't round any corners during all the tightening and loosening. The connections worked well. They come with nice, solid flare nuts to work with.
However! I found that the lines as shipped ONLY VAGUELY resembled my original lines and in no possible way would any of them (not even one of the shortest lines) just drop into the little retainers along their paths. All of them needed major manipulation in the form of unbending and rebending to get them to sit nicely in their respective grooves. It took several days of work and very sore hands to make them work out. And in some areas deep in the back of the chassis I barely got them to fit. If I didn't just happen to be an orthodontist where I bend wire for a living, they never would have worked and I would have been completely frustrated. Stainless steel is definitely harder, but not unbendable. My biggest complaint is that I halfway was expecting the lines to just drop right in. The other half of me was thinking that must be too good to be true. In my experience, a perfect fit as shipped was too good to be true.
But I was able to make it work, and the end result, I think, is beautiful.