a727 lines

Even if you could get prebent lines, I don't see how a person would get them in there.
I'm kinda in the same boat as you (headers and stuff in the way) and have pretty much decided to have them made with hydraulic style rubber lines and fittings.
I don't like clamping a rubber hose over a steel line because the ALWAYS end up leaking at some point.
It would work ok if you flared it and double clamped it as you said, but even then it will eventually start seeping fluid.
I can only trust full steel, or full hydraulic hose and fittings. (not a combination)