Help with rims

[scpbmxer01]

I have a 00 f250 with a leveling kit up front. I would like to know what size tires I can run without too much rubbing and what kind of backspacing I would need. I dont mind if they are wider then stock I was looking at 305/55/20 or maybe some 35's I dont mind if the tires sick out a little bit. if anyone has pics that would be great I am going to for sure get 20'' rims i just am confused on spacing and tire size any help would be great thanks.

Mike