Cap the old lines at both ends, and leave them there. Problem solved.
I have saved every old part that I upgraded, to offer to whomever may want to purchase my Turbo Coupe in the future. Another simple solution.
To remove the Flexi-Flyer aspect of all of the G bodies I have owned, I remove, reroute, and/or save all lines running in the frames "C" channel, in the open section, and weld 1/8" strap metal over the channel, making it into a box section. I do it very neatly and most peeps that have seen it, think it is factory.
Replace all of the body bushings first. Stock replacements are fine, as this mod makes the car more solid than urethane bushings do on a stock frame.
This mod makes the car hanldle like you wouldn't believe, and doesn't tear up the body like urethane do, and it is quieter as a result too.
It doesn't stiffen up the car as much as a 6 point rollbar, but then again, you don't have to climb over it, nor chop holes in the interior. I do the bar/cage/frame thing on my dedicated drag cars, but for the occasional "sub 11.49 and get kicked out once in a while" crowd, it works great.