Believe it or not, with those trucks, you can have the "best of both worlds" you are asking for, (depending on your def of amazing gas mileage) fairly easy. Here's an example, my Cameo, has a stock chevy 12 bolt rear out of a '66 Impala with 2.73's, an aluminum power-glide, a bone stock 350 Target Master Hecho en Mexico Long Block, Cyclone Headers, a single plane Holley Street Dominator aluminum intake, pure stock GM HEI, and an Edelbrock 600 CFM electric choke carb. When I built the truck I used the parts that I had available for it. As you can see it is absolutely not set up for racing. I get about 18 mpg out of this setup on the highway.

Please note, that I'm not recommending this set-up to anyone. It doesn't throw you back into the seat when you stomp on it and is actually kind of a dog IMO, a gear and intake manifold change are in it's future, but, I can do brake stands all day long in it if I want to. The thing is, that the rear end of the truck is so light that it really isn't hard to "smoke the tires". If your truck is a step side, it will be even lighter than mine and would make it even easier to light 'em up.

Good luck on your build.

Dutch