The BBK O/R H-pipe for LTs has been discontinued through BBK. I believe Brothers Performance said they had about a dozen when I last spoke with them - unfortunately that was a little over a year ago. I think I remember one of the guys at Brothers saying they purchased all the inventory BBK had remaning.
You can pretty much get any mid-pipe to match up to any headers if you don't mind a more customized installation process. But for a direct bolt in, the Mac h-pipe will not work with the BBK headers that well. The "cup" at the BBK header flange will not mate up properly to the Mac h-pipes inlet.