Yeah, I've seen those digital mileage computer deals and they looked pretty sweet but were expensive back then. Don't know if the price is reasonable these days. Fuel air meters are pretty cheap and accurate but you have to weld a probe into the exhaust to use one of them. Not exactly the same as a fuel flow meter but a good way to make sure your aren't running the
Engine to fat a mixture.
Does the bus have one of the big 2 barrel Holley or a 4 barrel? If you have a mechanical secondary (Double Pumper) 4 barrel on there, forget it your mileage will suck no matter what you do until you get a vacuum secondary carb and dial in the secondaries to open only when needed. Those double pumper Holleys are great performance carbs, but when you put your foot in them it is like flushing the toilet, they flow some gas. Also, a spread bore 4 barrel will give you a little better mileage if you can keep it from opening the secondaries at cruise. That big block in the bus should have plenty of torque to pull the load, but it may very well have to have all 4 barrels open to pull the load at highway speed. However, I'm sure you could tune on it a bit and squeeze some better mileage out of it.
There are a few cheap things you can do to get better mileage from a Holley. Mostly just keep jetting it smaller until it will barely run, then go back up a size or two on the jets. Jet the primaries lean and the secondaries a few sizes fatter so it will pull good when you have your foot in it. You can mess with the accelerator pump and put smaller less aggressive cams on there and that will help as well, but it will cost you acceleration performance for sure. Just as with the buggies, what ever jetting changes you do, only make one change at a time, and I would jet it under loaded condition since it will normally be used for a tow vehicle. A properly working EGR should help the mileage. I knew a guy that disconnected his EGR on a bone stock Chevy truck and it killed his mileage (that is with no other changes, just the EGR) . Once it was hooked back up it went back to normal gas mileage. The air pump will only hurt your mileage, it does reduce emissions though. Altitude can really screw with a carb as well. My old Camaro would not run for crap when I took it up through the grand canyon and reached those higher elevations. It ran like crap up that high. It was running super rich. I guess it was down on power from the air being thin up there.
One thing that will kill your mileage is the distributor. No matter points or HEI, make sure the mechanical advance weights and springs are in good working condition. If it has points get rid of them and go to an HEI. That is a good cheap mod. You might drop by the local county bus maintenance depot and ask some of the old timer mechanics. They might have some mileage tips specific to your bus that could help. Some of them old timers really know their stuff and can be good people to know for parts resources as well. If I can think of any other things I'll post them. I have a few books on tuning Holleys if you want I can send them up to you. I don't know if you have experience with them but they are real easy to work on and figure out once you have messed with them a little bit.