I just purchased a 2007 F350 6.0L diesel and it needs new tires. It has 325-65-18 cooper tires now. Not sure if the wheels are stock, or their width. Has a 4" lift kit. Wheel wells have been removed. No tire rubbing issues. The tire dealer suggested going with 17x9" wheels because there are many more tires available in that size. I'd like to get a larger tire, in the 325 ballpark. Any feedback would be appreciated. I have no experience dealing with wheel offsets, so that was a concern.
That tire dealer is full of it. There's no reason to go to a smaller wheel, there are plenty of sizes available for 18" wheels and most have a higher load rating in 18". 18" is a stock wheel size for these trucks, but to know if they are actually stock wheels we would need to see a picture. 325/65/18 is not a stock tire size, the stock size would be 275/70/18. 325/65/18 is roughly the same as a 35x12.50, I don't know that you could go much bigger with a 4" lift. You might be able to squeeze a 37x12.50 on there, not sure though.
If you are getting different wheels, I would actually suggest the opposite of what the tire dealer told you. Rather than go to a smaller 17" wheel, you will want to go to a bigger 20" wheel. Once you start getting over 35" tires, the 17" and 18" tires don't have Load Range E ratings, where 20" do. If you are going to be using the truck for hauling/towing, you need to have Load range E tires.
There are plenty of tire choices for an 18". Go to a different dealer, that doesnt want to sell what he needs to get rid of. I hate when people lie just to get rid of a crappy inventory choice.
If the stock wheels are still on there, I would definately get wider ones specially if you want to go even bigger. Sure, the bigger tires fit and alot of guys run 35's on the stockers, but the tires will wear aweful and ride even worse. I bought a combo set from an online drop shipper in my area. The problem with that is not being able to lay eyes/hands on them before the purchase. I asked guys I saw driving around plenty and advice on here before I made my choice. There are a few great threads explaining rim offset and what fits with what lift with pictures.