Daughter needs Gas mileage from 273 commando
I wouldn't recommend the large 4 barrell injection on a small engine...
You can install various injectors. They are standard Delphi TBI injectors. I have one Commander 950 with their smallest TBI (600 cfm, I recall), which might be good for my 273. But, I was planning to use that on my 383. For the 273, I have a Magnum manifold (MPFI) that I hope to use (yes, I know the bolt angle issue).
The 4 injector TBI's could be more efficient than say a Holley Pro-jection 2D TBI since you normally run on just 2 injectors until the secondary throttle plates open. Problem is most people install the biggest injectors they can find, then complain. Same deal with carburetors. Don't over-carb. It doesn't give more power. Match the carb to your max airflow and be honest about that. For a 273, a 370 or 450 cfm carb is a good fit, but they are hard to find. The Holley Economaster is one.
The best way to get good mileage on the highway is to follow a semi. I did see the MythBuster's episode where they supposedly discounted that, but my 2002 T&C 3.8L has an instantaneous mpg indicator and I get 33 mpg at 65 mph behind a big rig vs 24 mpg normally. I don't follow too close. Indeed, other cars sometimes pull between me and the truck. I usually only do that on a long drive to LA, when I find a truck or motorhome that doesn't have the 55 mph governor common here.