i saw some photos where they are right after the headers and some after the X itself. i always figured everyone runs them right before the X but i dont think it matters
I think the proper way to install them is to mark down the pipes with some type fo a wax crayon. Then run the car and were the wax is melted off the most is the best spot. At least, that's the way I recall it being done back in the day.
Regardless, I don't think you will gain anything, and probably just lose all of the bottom end you have now.
open headers sound like ass I think...I'd put the cut-outs right before the mufflers. Closed cut-out = normal stang exhaust, open cutout= no mufflers and probably plenty loud and you'd still have some backpressure because of the mid-pipe