power vs. ability vs. mpg

Well you cant have it both ways...If you want your car to seem to have more power a rear gear change would accomplish that but it will hurt your MPG especailly at highway speeds. This is very basic and not exactly accurate but if you'll think of your engine as an air pump. More air=more power and less MPG. Less air=less power but better MPG. If you want to keep the RPMS down at 45+ look into changing your transmission to an AOD or a t5 which will give you an overdrive gear.

When my 289 was a stone stock C-Code 2bbl with 2.80 rear gears, it averaged 16mpg city and roughly 20mpg highway. This was with fresh everything (point/condenser, wires, plugs, filters, perfectly running Autolite 2100, etc).

As a 2bbl car, it ran out of breath just shy of 5000rpm.

The only difference between your A-Code and a C-Code is the 4bbl and slightly higher compression, otherwise everything else is identical. In a good state of tune, your 289 should get similar mpg to what I said mine got. If it already is getting those kinds of mpg numbers, then that's about right.

If you want your car to accelerate faster and feel like it has more power, as has already been said you mpg will go down, assuming you use traditional horsepower improvers like aftermarket cylinder heads, camshafts, headers, etc.

There are a couple small things you can do to improve both mpg and horsepower.

If you still have point/condenser, replace them with a Pertronix Ignitor or Ignitor II or any similar electronic ignition module that fits inside the distributor cap. This will improve spark energy and won't wear out like points, so you get a little bit more complete combustion which will improve both mpg and power (maybe by 1-2hp, maybe).

You can also replace your stock mechanical fan with an electric fan, this will reduce the accessory driven load on your engine, freeing up horsepower and will help mpg a little as well.