Safety is very important to me and hopefully to you, as well. My OEM on my 08 F550 were 225. The weight I carry and the rough gravel and dirt roads just were not comfortable traveling down with the sidewalls bulging as much as they were. You mentioned that you measured and you have 2 inches of clearance with your present tires, but you should be aware with the 245 the face section of these tires are 1 inch or more, wider than the 225. So your clearance is going to be reduced. Then you will have the camber of the road and side loading these possible 245, your sidewall clearance will be even more greatly reduced.
No, I don't recommend that you put 245 on your OEM rims. Also, using spacers are just that, a "fix." I would never wish to reduce the thread surface by reducing the thread section holding onto my rims.
I bit the bullett and purchased Rickson Wheels, as well as new tires. I did offset this purchase by selling my OEM Tires/Wheels, including spare and front bumper. (Swapped it out for a Ranch Hand Bumper and 16.5 Warn Winch)
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