You can probably deviate from the stock size slightly with 255/55/18 (~3% shorter) or 265/60/18 (~1.5% taller), so check out those choices as well (a lot more tires to choose from). Some have run snow tires at 235/65/17, which is 1" shorter than stock, with no problems.
As for the GrandTrek 8000, its a very old design. The Pzero Scorpion has been universally scorned when it comes to snow traction and treadwear.