I put a CC on a RCLB also because we have kids, and it was a lot of work, had to cut 30" out of the box and use a 3" bodylift to clear the frame as previously stated. It took about 23 hrs for me and a buddy just to do the metal work on the box. It was a fun build tho.
I do know a guy who fit a CC on an ECLB frame with a stock short box, but also needed a bodylift to clear the frame. It was alot less work than mine.
Try to find a complete truck for your swap with a good interior, I can't find any interior trim pieces for the rear anywhere new, had to scavange all the parts outta used rigs. And still am looking for the rear lower door sills. Good luck with your build.