318 2bbl to 4bbl: Is it worth it this time?

If you go to a square-bore carb mileage will probably get worse. It will only improve if you go to something with smaller primaries than the BBD you're currently running, such as a ThermoQuad (which is what I'm putting on the 318 in my '70 Duster on top of an '83 iron pickup truck manifold). You should also be getting MUCH better mileage than that, I've heard of people getting upwards of 20 MPG in their stock 318/727 B-bodies. I guess you keep your foot in it then.

I want to go to dual exhaust on my Duster too, what did you do for your exhaust? Muffler shop?