best size 2bbl carb or small 4 bbl for my 318?

650 AVS carb? Or is it a AFB? AFB's are 600 unless there the 625 cfm Emmissions carb, but I can't see where the extra 25 cfm comes from.

Anyway, that carb is just fine for mileage and should simply be jetted lean. An 02 sensor would help dial it in best. Next would be rear gear ratio followed by an overdrive transmission. Then taller tires.
That would end the mechanical part of mileage getters.

Non moving mileage getters would be;

Free flowing air flirter and if the houseing was open, even better.
You allready have a good carb. A 500 wouldn't really help any and even if you found mileage with a 500 Edel. Is it really worth the expense?
Perhaps a 4 hole spacer under the carb.
The intake is good.
Heads leave stock. A valve job would help, but it's also a bang for the buck issue not really worth doing.
Exhaust, dual 2-1/4 for a stock or near stock 318.
Headers do well as well.

Electrical;

A Multi spark box is excellent at adding mileage.
Also, new wires, I like MSD wires, with the lowest ohms per foot and new plugs.
Adjust spark delivery for max. mileage. (Advance it, but be aware this may require a higher octane to run. The cost of gas is a marginal increase on the wallet vs. a possibley slightly better margin of mileage. But it is very possible.

Liquids;

Synth. liquids help. It's a small gain, but just keep adding small gains up and you'll find another mpg in there. Also, cool fluids do the job better and longer. Deep trans pan/oil pan and external cooling lines do the job well. (OK, a bit silly & hard for the rear end, but it's doable.

Parasidic drag;

Can you get rid of some? Clutch fan, P/S ???? can you live without it. This is another bang for the buck issue on the P/S, but not so much for the electric fan.
Taller tires I mentioned, but also keep them thin. Over inflated tires have been proven to get better mileage.
Lower the car, less drag.
Lower the weight, less energy spent moving a heavier car.