Towing a fifth wheel with a 6" lift. Will I have issues?

I'd call the guy and ask him how high the sides of his pick-up box are and then measure yours. I think you'll have problems. You might have to dump the lift kit and tires or raise the 5th by putting a 4" or larger piece of tube stock between the spring hangers and the frame rail. And that's not going to help the weight of the 5ht any.

Bill