Sounds like you have a catalog or web site to order wheels and tires from. It's a great way find a good deal on a wheel/tire combo, the downside is if you pick wrong you pay the freight to return them.
There is no simple way to go bigger/wider. Measurements from tire specs can be used to try and determine fit but it usually comes down to putting the package on and finding out if it will work.
What I have done many times is put some old thing which is close to the size I want on the vehicle and take measurements from it, adding to it what I need to see if the tire I want will work.
The rears are always easy but the fronts are hard because of the turning angles along with the height changes.
The suggested tire sizes are a working size, but it sounds like you changed the ride height on your truck so that will have to factor in.
Find a tire store that will mount up what you want and try it.
I would think some where in Chi-town you could find a dealer who would work with you on that.
You might have to buy from him, but in the end it might be easier
Dan
