Allen,
I ended up swapping in a 2004 lq4 into my truck because the 4.8 had a bad castech head on it, so i had yall do a retune to run the 6.0. Well, it runs fine and all but the mileage is absolutely horrible. This is our family ride and we occasionally tow a boat. It doesnt get run hard at all, no racing or wide open runs. The best i can get out of it is 14 MPG. A 6.0 in a half ton truck and 4l60e im thinking should do better on fuel, and most that do the swap are getting much better (except for the modded ones of course). I had a 30 year GM tech at work ride with me with a tech2 hooked up and fuel trims stayed between +8 to -8 at light acceleration and cruise. He said everything else looked to be in spec, MAF, front o2's and all. I emailed Erica to see if the tuner could do anything with it but she wants me to send a bunch more scan info, but would be hard to do again, but as i said, the tech said all was fine..