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IMO, If you're going to go to the trouble of replacing the lines - the only way to go is SS - the added cost is well worth it!
I bought a set for my 68 Camaro some time ago, & just recently for my 68 vette from Classic Tube - every line was bent perfectly (they use CNC tubing benders & the original GM specs), and fit perfectly no matter how much crap they had to go around. It will be helpful if you can get the car up on a lift to install the main line from front to rear though.
The main line from front to rear of car is shipped bent in half to reduce length of shipping carton - it is a large radius bend and easily straightened.
For my $$ I would replace the brass blocks also (and did on my 68) - again, if you're already under there changing out lines you might as well go to the extra effort and replace everything (don't forget the brake line clips while you're at it).
Another good vendor for lines is Inline Tube - I've seen their lines - same quality/mfg. techniques as Classic Tube. I'm going to guess that like most things there are only so many vendors for a given part and that everyone else just sells them from their catalog & marks up the price - I ordered mine directly from Classic Tube.
Cam, I did both my '68 convertible & '75 coupe a few years ago with the "body on". It's not near as bad as doing the fuel line "body on" :yesnod: I myself do not recommend stainless. It is a much harder steel and I've heard horror stories of these lines leaking on people because they are not "soft" enough to form & tighten down. The "OEM" steel lines I got from Classic Tube to this day and did a few years ago have no rust and look like I did them yesterday. The main problems are where the rear main line ends in front of the left trailing arm pocket. Very limited area to get the flare nut tightened into the receiving socket-can be done, just not easy to get to. The other main problem is getting that front passenger side line into place. You have to feed both ends from the front of the car towards the rear as you carefully unbend it a little bit to get the "V" part of it to go "up & over" the sway bar area. Just don't kink it. You'll also have to carefully snake it through between the lower radiator hose and frame. I wrapped a little duct tape around the ends to protect them from getting any dirt into the lines as I did this. It's not real bad, just work carefully and take your time & be PATIENT. Don't remember if they were folded in half. The fuel lines I got from them were but they were done in large swooping bends, so they were easy to straighten out.
I purchased a complete kit from Zip for my 74 BB, w/ss lines. The only problem I had was the front crossover line. I wouldn't recommend an after market prop. valve because of the poor quality.