Best possible mileage?

ok, so i wanted peak mpg's w/78 b100 van, has /6 od 4 speed. because of of hilly ny highways makes for alot of down shifting. at 4500 lbs takes some pushing. getting 16 mpg. i feel there's more to get. wondering should of went 318? figured w/6 it'd be better and last longer wise. i will try aj's timing trick,and get back. also putting 2-1/4 exhaust n free up so flow.