I wouldn't have any reservations about using the Surly LHT on the Great Divide route. Especially the 54cm size with the 26" wheels. Most of the trail is apparently dirt road, with 2 track jeep roads, and some singletrack. I thing a fully rigid steel mountain bike, or a durable touring bike with 32c or 38c tires, would be best. Good 36 hole rims would be important. I don't think you would get much advantage out of a cross country suspension fork on a tour. Maybe a little on the washboard roads.

I'll be touring some dirt roads up to Cripple Creek, CO on a touring/cross bike, 700c wheels, with a Bob trailer. If that goes well, I'll try the Great Divide route. Not sure if I will use panniers or the Bob trailer.