I think the 9" is a dual diaphragm (please confirm 1st), in which case it should work as well or maybe better than the 11" which is a single diaphragm. Looks a lot better than your valve covers self-clearancing on the booster too.

Another option, that is expensive, but provides a huge improvement in brake pedal response, is the hydroboost brakes. Packages much easier, cleans up the firewall, provides great pedal feel, and works no matter what engine vacuum you have.