I also replaced my alum tank with poly tank (27 gallons). My aluminum w/ baffles...just barley got it out without having to cut deck. New tank came with top brackets that mouted to top of tank and then screwed into supports on stringers. Working out ok so far. I think you should be ok with 55 gallon poly tank. I would be concerned with anything with larger capacity without baffles though!
here is pic of my tank
http://i271.photobucket.com/albums/jj157/fdheld34/th_100_0861.jpg[/IMG][/URL]
