The rule of thumb I use is add 10mm of fender width to the tire width you want to use. I don't have a 26" wheeled LHT to test, but the Surly website says you are good up to 2.1" tires + fenders so I'd be confident the 60mm Cascadias would work. If you want to be 100% sure - I'd ask this question on the Surly LHT Google Group.