I work at a GMC dealer in service and the 6.0 gets alot worse gas mileage. Its just my opinion, but I would never have a 6.0 over a 5.7. I even have customers that would sacrifice a little power to get the mileage of a 5.3 or 5.7.
My '02 Formula has gotten 26.8 mpg on the highway. Yes, I know thats in a lighter car and 6 speed, but I bet a 6.0 in an F-body would not get that.