16MPG is not bad for a full size chevy truck, they are nortious for getting crappy gas milage anyway.
A high flow cotton air filter like a K/N, 2 barrel carb & highway gears would be my first thought.
Keep your tires aired up to the MAX setting, always.
Do you have a overdrive trans? If not put one in.
Snythetic oil in the engine, trans and rearend is worth a few.
I'd also stick a giant vacuum gauge on the dash and drive to maintain the higest vacuum at all times.
Goodluck, Cp