True, the smaller primaries will help with fuel economy at low throttle positions, but that all goes to hell when you push the pedal to the floor. The 2.8L simply soes not have the displacement to take advantage of the 390cfm that the Holley offers. It may allow the secondaries to open at WFO, but the poor air flow through the carb won't allow decent atomization of the fuel, thus killing both economy and power output.
In the end you could get the same effect you are illustrating by simply using a smaller carb to begin with. The Offenhauser intake is a nice unit for sure, but unless you are building a 2.8L to scream (7500 RPM plus), you simply won't be taking advantage of the flow capacity that the Holley 390 offers. Hell, even at 7500RPM the 2.8L is still only chugging 371cfm of air. Again, that is crude math assuming 100% volumetric efficiency and doesn't take into account things like barometric pressure and such but it illustrates my point all the same.
A 2.8L that is built to utilize the Offy intake to its full potential wouldn't be suitable for use in a truck. Hell, in my area you could perform a 4.0L swap for the same price and have much better results.