"Gen 2 BMC on, still rubbing"So do you now have the original 7/8 M/C with the original booster you had before the conversion?
I had previously wondered if push rod clearance between the booster and the M/C may have been your problem, but if you now have the original booster/MC combo that presumably worked ok with your old brakes before you started the swap then the problem is probably not that.
(When fitting different M/C's or boosters you are supposed to check the push rod clearance between the booster and the M/C. The service manual describes how to check and adjust that, along with the brake pedal play).