What about simply making a tapered spacer to slightly angle the master away from the plenum?
I would think there would be enough leeway in the pedal rod to accommodate.
Or, a cool setup I've always wondered bout would be to make a simple adapter to the factory type angle bracket to eliminate the booster and just mount the master straight to it.
You'd have to twiddle with making a custom actuating rod for the master side, but I would think with the parts available that would not be too hard either.
And you have almost all the parts already.
Never tried it, or even taken measurements to that end. So if that doesn't fly for some reason, what would that reason be?
Paul