Tristan S.-
A couple of things you might want to keep in mind: One, that deck subframe isn't perfectly rigid, and the deck can swing side to side on uneven terrain, while mowing along brush, or can be moved A LOT if you bump a tree while mowing around it. This can force one of the rear gauge wheels into your tire, which won't be very good for your tires. Two, 27" rear tires will make the tractor sit leaning forward, which could do strange things to the geometry of the steering and make it harder to steer, not to mention that the deck will be far from level, and you may have to space off the rear QA latches significantly to make up for the difference, which may or may not be feasible, depending on how tall those are. You'll most likely have to space the fenders up for 26" or 27" tires, and you most likely won't be able to use chains with those sizes either, but I imagine your objective with the ATV tires is to eliminate any need for chains, so that last thing won't be a problem for you, most likely.