With the 9" booster I`d try a 1.125" master and see how you like it. If the pedal travel feels good but the effort is too high or if you want even more stopping power pitch the vacume booster for a Hydratech unit.
We worked with Paul and Baer on the brakes. The `Burban has a bizzare pedal ratio so everything was a little different,we`re actually running a huge 1.25" bore master on there. There are still a lot of little details to finish up on it before I can post pics but you can bet I will! It`s a big bad ride.

Mark SC&C