If you don't have a leveling kit, and want to run 305s, I would think that you probably would need a negative offset for sure... maybe -12. I just put on 305/55R20 Cooper Zeon LTZs on Fuel Hostage Wheels, with a +1 offset, but I have a 2" leveling kit on the front of my truck.
I posted some pics here (last post):
As you can see, it's not so much the height but the width... just enough room for back spacing. You may be able to get away with a neutral offset with no level, but you may need a spacer. If you look at the picks, even with a neutral offset (+1), they are going to stick out a bit; about a 1" or so on my truck. I did use to have 315/75R17s DuraTracs on a -12 offset wheel and they stuck out probably 2 1/2" - 3". So yeah, if you do have to go to a negative offset, it's defiantly going to stick out more than mine that are pictured.