I tried 42mm tires on my 700c LHT, the fender mounts rubbed on the tire. I could have bought new fenders with different brackets but I instead just gave away the tires to a friend that did not have fenders on his LHT. Plenty of room for 37mm.
Surly Long Haul Trucker, Surly Karate Monkey, Schwinn Tempo (first bike), Schwinn Woodlands adventure/trekking bike, Trek 700 backup touring/bad weather bike
I'm running 700x40c Vittoria Randonneurs on my LHT and they run very true to size. I am building the bike up and have SKS P50 fenders, the fronts clear with room to spare, the rear is a little bit more snug.
I may just lose the fenders so I can run a 40-42c knobby tire for gravel road touring.