You know this probably isn't at all helpful, but I've always gone the other way for float tube fishing, preferring shorter rods. In a tube you're generally just trailing your line behind you basically trolling or casting to risers that you can get amazingly close to. Neither requires long casts. Then once you do get a fish hooked and close to you it's much easier to land them with a shorter rod. My favorite tube rod has always been an old 8 1/2 ft 4 weight with a rather moderate action, but I had an old 7 1/2 ft 4 wt that I really liked for a while. Like I said, probably not helpful at all, but I guess there's always two sides to every issue.
"Perhaps fishing is, for me, only an excuse to be near rivers." - Roderick Haig-Brown