First you are correct you can't run a different size spare but, I know caleb did it and was fine getting to a tire shop. If you're not rubbing now just look to see how much room you have left and judge from there. With small mods like the pinch weld mod and reforming the plastic both have write ups on here you can go pretty big and have no issues. Running those specs ( 265/65 r18) in the calculator I think you'll be fine doing it especially with the the previous mods mentioned being done. Keep in mind you'll probably have some rubbing when you turn the wheel full lock which can't be helped without spacers or new wheels. Also don't forget if the tire clears the knuckle it will always clear no matter how small the space is the tire knuckle spacing never moves. My tire guy was worried when he let the jeep down the tire would hit it LOL I told him that space never changes if it clears my a fraction of an inch it will always keep that distance no matter what.