The diameter of the 245/45 X 18's will be about .70 inches SMALLER then the stock 225/70 X 15's. So they should fill the wheel-well about the same as stockers, on a stock suspension. The thread width will be a bit wider, but not a problem. Your speedometer will be about 2.5% fast. At 60 MPH INDICATED, your true speed will be about 58.5 MPH.
I've got 235/50 X 18's on mine. Sorry, I had already lowered the truck when I went to 18's.