Larry's Carson City Pantera Parts Connection not only has premade stainless lines, he has 'pre-gutted' proportioning valves in stock- both early and late type.
Note that with bundyweld (stock U.S welded-seam lines), double flares are essential. With stainless lines, they are made seamless and need no double flare. In fact, attempting a double flare may crack a stainless line.