I'm using BT's with BBC Exhaust. You might want to do what I did. I took a set of CJ headers, chopped off the flanges, and welded up a set of flange/stub assemblies I bought from "headersbyed".
www.headersbyed.com. I had ed custom shape my tube stubs so his ID was precisely my OD. Then, I fit the parts up the way I wanted them, tack-welded, removed, and finished the job off the car. It was easy; not to mention affordable.
http://www.racesystems.com/cobra for my project.
Byron