I think your current setup will get the max MPG as it is, 3.23 gears and stock cam is about as good as you can get for mileage (taller gears would do better on the highway but worse around town). I would just do a full tune-up (ignition parts, timing advance, carb tuning) to make sure it's running as efficiently as possible. The dual exhaust is also already helping with MPG compared to the stock tiny single pipe.
You could add a 4-bbl carb and a mild cam for more power but that won't do anything for gas mileage, it'll either stay about the same or go down slightly. A new small 4-bbl carb with small primaries might do better than a 2-bbl but it's a toss-up IMO.
Or dump a few grand in the car for an overdrive transmission and EFI lol...
