Carb "cfm" has nothing to do with fuel economy...proper carb matching and tuning does though.
You are not gonna get anything approaching "economy" when running a tractor engine inna vehicle that has a typical curb weight of 4200>4600lbs. And the frontal area of two sheets of plywood side by side. When these vehicles were designed and produced, economy was the last thing we cared about. Load capacity and ability to work were the criteria. These are not fords or chevrolets.
Normal milage for a pickall will be in the 10>13mpg range, maybe slightly more if you drive inna totally anal-retentive manner using only a vacuum gauge as your guide. Milage for all three sv engines inna squarebody will be near identical. We drive this shit in order to make a statement about political correctness and the lack thereof. And if it infuriates owlgore and obamarama, then blow yore tires up to 80 psi and add another tenth of a mpg until the maypops pop.
Line-installed ac was available on every platform, as well as dealer-installed ac "kits". Then there were the various "aftermarket" ac systems that were fitzalls...same as what some vendors sell for "kits" today but are not dedicated for a specific IH model.