Yeah I expect it will squat a bit when you fill it up. That's about 220 lbs. of fuel
Actually it's only about another 100 lbs. of fuel vs filling up the 20 gallon tank.
I worked a long while on the tune to find that gas mileage. They are all capable if setup right and tuned properly. The problem for most people is finding a tuner that is willing to spend the time on it, with an eddie current dyno that can run steady light loads for part throttle tuning....then there is the cost of paying a tuner to do something like that,,,because it takes hours.
The sweet part with this EFI is being able to play with different fuel cells in various load conditions and commanding exactly the AFR you want, when you want PE, etc...
The stock truck tunes are lame (worse than the cars), with nearly zero timing in most cells, even under full load you'll find maybe 16 degrees total
The throttle tuning section has alot to be found, seem there is alot of delay built in that can be tuned out, things that affect engine braking, and then the best part is playing with DFCO which cuts your fuel off when decellerating. Playing with these tables making it more aggressive usually finds 1-2 mpg. I won't bore anyone with details as you can literally spend days tuning and logging.
Neat build thmb2g, nearly identical to mine. I've heard the same thing from other people that you reported about your tank. Can't wait for it to get here (LITERALLY) as my wifes car is having issues, with my blazer down I'm having to resort to driving other vehicles that I don't want to really drive everyday. I'm anxiously waiting.....pics will follow.