I wouldn't worry about staggering the secondary springs. The joy of getting it to idle and part throttle will temper any ideas of getting carried away with the tune. Power valve selection will be important also.

As far as fuel mileage? I've only run 2 dual carb set-ups, one a tunnel ram 350 and the other was a cast iron (flat) 400. Maybe I got 11 mpg's? Once?