Will they fit?

That's probably the easier way to go. You can still pretty much max out the wheel wells in the back with that combo if you have the stock spring locations. Some guys do get away with running a 255 in the back and that would take a 9" wide rim, but not all cars can manage that width in stock form especially if you're lowered. Mixed bag with a 255, some cars need trimming of the quarter lips, some say it works fine, but if you're lowered the odds are a 255 won't work anyway so the 18x8" will get you to a 245 and that's a better bet if you don't want to trim back the quarter lip.