How fast have you gone with exhaust manifolds?

I'm running a bone stock 67 GTX, 440 auto, 3:23 sg with all original parts, and have had a best time of 14.06. Probably could have dipped into the 13's one time but I had to lift to keep from breaking out. I am not looking for anything other than a good time at the track with family and friends, so I'm happy with it being all original, but I would have liked to dip into the 13's that time, but I didn't want to lose. Have fun all!!! Geof