Andy,
I ran my B12 with stock exhaust and a HS Stage 1 kit with great success.
107.5 mains, stock pilots, mixture screw out 3.75 turns, hole in air box, cut springs, 5 degree advancer, and 4 shims under the needles.
I am at 11 feet above sea level.
Try it with stock pilot jets. It will run great with stock exhaust.
Good luck,
Dave