what rear would you go with

Your current recipe of uppity cam, high stall converter, and 3.91 gears is much better suited for drag racing. If you change to a taller rear gear, that cam and converter will need to be changed to match it in order to get better fuel economy, and any reasonable drivability. You may even want to consider a vacuum secondary operated carburetor as well.

One can purchase a lot of fuel for the price of rebuilding a drive train, and performing the engine modifications need to convert your car into a more efficient highway cruiser.