Truck is a stock f350 4X4 (diesel) and I need tires, looking to upgrade size and wondering what the largest is I can fit? I think they are 275/70/18s now

 

I ran 295/70R18 on 8" wide stock wheels, no problems. Should clear up to a 35x12 with stock suspension based on the 34x11.50 fitting so well, but I wouldn't go any further than 12.5"/315mm wide because of the wim width. All the 18" wheels that year had the same 8" width and 40mm inset, so even though this isn't the KR wheel the fitment is the same.