Install a temporary vacuum gauge and tuck it under a windshield wiper so you can watch it while you drive. I think you'll be surprised how often it stays down in the sub-10" of mercury "power" range with the metering rods in the full rich position. Lower gears allow the manifold vacuum to stay high enough to keep the metering rods pulled down in the leaner ranges.

With an OD trans, performance and fuel economy would probably be better with a final drive ratio somewhere in the low 4's.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!