what would be considered good fuel economy on a 6.0? i have several customers getting upwards of 800+km on a tank of fuel under sensible driving conditions. my brother inlaw did pretty much the same thing, bought an early '04 for cheap from auction with just enough warranty left. i did the headgaskets, installed studs(no o-rings on this one though). from there i installed a banks intercooler, piping and intake elbow, an airaid intake, 4" exhaust, and an sct with a customized tune. his truck gets an easy 800km to a tank and has about 80,000km on it since all this has been done with no signs of anything yet. he also runs the pm-22 additive with every tank of fuel. the custom tune was very mild, i would be surprised if it even added 50hp, more so just a "control" tune as i like to call it.
as mentioned of course this does come with some increased emissions, but i did leave the egr system intact on his truck as well as the cat.
i do alot of this stuff, i do however make all of my customers aware of warranty implications and even make them sign a waiver(which would probably mean squat if it ever came down to it), and rarely have any problems. there are the odd extremests that break stuff on a regular basis, but you gotta pay if you wanna play right.