Delayed response I know, going to try and stay active on here now. Basically, I would use the truck to occasionally pull a camper or haul cars but still cruise around a bit. Wanting to do anything to improve fuel mileage which I know still won't be amazing. Also want to increase power to the 400 HP range via head work, tune, e-fans, headers, injectors and any other quick bolt on mods. Does not have to be emissions compliant. Truck has factory 4l80e, 4.10 gears with posi 2WD.
Well, 400 HP, and 410 gears, that's asking for bad MPGs, but let me make you feel a little better. I just bought a 1979, Corvette. It has the OE L82, 350 in it, with cam, Edelbrock heads, Edelbrock intake, long tube headers, Holley sniper EFI, Holley distributor, roller rockers, and I admit, that it is running a little rich rite now, but I filled up last night, and I'm getting 6.7 MPG. Keep in mind that it has its OE Muncie 4 speed transmission, so there's no OD, but 6.7, is bad, no matter how you slice it. My 97 Silverado, only gets between 10 and 11MPG in town. It is the Vortec 350, with 373s and an Eaton truetrac. It has shorty headers, cats deleted, tuned with a jet programmer set to max performance, electric fans, big 3 upgrade, but really it has 2 extra grounds, so the big5 up grade. And a ton of mods besides those. The internals of the engine, are OE though. If you want good MPGs, one of the biggest things is small tire size, ( like the OE 15 inch tires), and rear end gears. If you can get the RPMs down, that will give you better MPGs, but you will sacrifice that put you back in the seat performance, and you will loose pulling power with a higher gear ratio. 273s will give you grate MPG, but your truck will be a dog.