You can use 18" wheels on the S373. The Buck came with them and has the identical transmission to the S373 to the best of my knowledge. I'm not sure about fender clearance but I'm guessing it would be ok. Iseki did this too with their TX series tractors. Some will argue about the differential between the front and rear tire diameters causing undue stress on the 4wd transmission components (obviously there is no worry about tire diameter mismatches for 2wd). I don't believe this is that big of an issue when talking about 16" vs. 18". It could become a problem if they were drastically different from someone's "custom" adaptation of different tire diameters. The main thing is to limit the tractor to 2wd unless you really need 4wd especially on high-traction surfaces (asphalt, concrete). There is always at least some presence of differential no matter how close one tries to get to the ideal ratios due to tire wear, pressure, and other factors that affect diameters. Problems are minimized with this by using 4wd only when needed.
