Other states are now doing smog testing (including some places here in Texas), so you do have to be aware of what mods you can do, what parts are C.A.R.B. exempt or approved, and so on. With the stock 2-bbl, you should around get 8-10 mpg city, 12-15 mpg highway. The Bronco weighs nearly 6000 pounds, and even the 351W is low on HP (139HP for the 1985). You will get the best mileage with a properly-tuned engine, so that's where I focus most of my work. Regular tune-ups, filter changes, fluids, etc. are essential. A minor vacuum leak, misfire or electrical problem all rob you of power and mileage. If you modify the engine for more power, start with the exhaust system. Many older trucks have clogged catalytic converters, and much of the emissions equipment is not working. My approach has been to keep as much of the pollution control as feasible, both for cleaner air and passing inspection. I have repaired most of the original items, even some that are discontinued. I don't advise removing all of the emissions stuff and the cats, but am pleased with the Flowmaster muffler.
Whew, I'm winded!!! New exhaust, engine tuning, good driving habits to get best mileage.