Has anyone here done some successful mods to the 4.3l V6 to improve its performance and gas mileage?

I just purchased a 2004 Silverado 1500, short box regular cab. It's a very clean low mileage (50k) truck with a 5-speed manual trans and a 3.42 rear axle ratio. Driving over 500 miles at 65-70mph on the highway netted me a whopping 18mpg. I was expecting more. Plus, this engine seems a bit weak and like all good Chevy engines I'm sure it has some potential.

The truck is all stock with the exception of a dual muffler system that splits just behind the single stock catalytic converter.

I'd like to hear from some of you what you have done to improve this engine in terms of both performance and gas mileage. I'm not afraid to toss a cam in it either, if needed, or to rework the stock heads. I'm not looking to turn this thing into a fire breathing street truck, but I am sure that it can be improved upon in some key areas from where it is now.

And for those who want to post and tell me to just drop in a v-8......that mod is beyond my time availabilty, pocketbook and needs.