How would I get better 6 cyl gas mileage?

[Pumkin Truck]

K, here is the deal, I got a better job offer that pays hell of a lot better than my current job, to be exact, it is thirty miles away. My six is all stock from the ugly air cleaner to the oil pan, My question is, should I buy a 4 spd tranny to replace my 3 spd chicken leg since it would be all highway driving? I am really debating this. thanks for all the help. Oh yeah would it be a direct bolt on with out modifying my drive shaft? thanks