Expected rear end shift?

[clemsonteg]

How much rear end shift can I expect when I air out? I will be installing a longer track bar and centering the rear end at ride height. I am a little concerned because it when I test fitted my rims this morning (20x8.5 5.5" bs) it looked like I only had around an inch between the inner wheel tub and the rim. I was going to run 255/40 tires all the way around, but that doesn't leave much room for the tires once aired out. I won't be slamming the truck on the ground, I'm not even installing a notch for now.