I did a quick check on 1010tires.com.. 225/40R18 are already bigger than the oem size, so 235/40R18 are quite large, about 18.3MM larger than OEM size, plus they're a lot wider. Tire selection is limited, but a 245/35R18 might have been a better choice.
Anyway, you might be able to mold the inner fender with a heat gun. There are a few fender rollers around the city but most people are reluctant to lend it out. I guess Princess didn't renew their order for them; too low of sales.