Really, that is great mileage. My 351 gets around 10-11, almost all around-town driving. Highway, I can get 15 if I keep the speed under 100
Set the cruise at 70, get 15 pretty consistently.
But if you want to be sure, check the air filter, replace if necessary, same for the fuel, but you can't really check that, so replace if you haven't done so already.
A TB cleaning job is a little intensive but can yield good results in performance.
But really, if you want great mileage go get a 4-banger import (or a 3-banger Geo lol). These trucks were made for work, not cruisin' the interstates. People don't buy Civics to haul boats, people don't buy trucks to commute. But 16mpg is very good, your motor is in good shape.