I did it on my truck when I went to 63" rears. I slid everything forward on the frame to the length/wheelbase I wanted to end up with then I cut the bed to match. Mine is in the middle of a long bed and shortbed. I wouldn't have even thought about it if I had to cut and weld the frame, at that point just get a shortbox frame. If you have enough lift and setup bumpstops correctly the frame being lower won't hurt you except for a possible limit on up travel. My tires would personally hit the top of the bed's wheel wells before the axle could even touch the frame and I'm only running 35's.