No 18" won't fit the Paralever...but if you want spend money,get a new Rim (excel or Baer) with thicker spokes 4 mm more left and you can use the Michelin Desert in 140/80 17"
Why thank you that's what you call Punishment from the lord and master of this jaff for trashing the GS just thinking of undoing the knots if lilith promises to fix it