What do you mean by heavy seas. If it's around 20 knots with chop then...
I'm guessing the old Fanatic is much narrower, thus designed for a smaller fin. The narrowness also cuts chop better. The Futura 144L is for light wind, big sails & prefers flat water & big fins. I'd say the 5.4 is simply too small - it's like trying to run your car in 1st gear at all speeds. The 7.3 may be on the smaller size but ok. For stronger wind & relatively small sail, move the sail forward in the mast track a bit to help keep the nose down. Skill & good aftermarket fin will increase the upper range of the board though the board will have it's limits in that when it gets too windy & choppy, it will be a handful inspite of fin size.
For other specific fin options, I'd suggest your best source is contacting directly a specialized fin dealer.
Last edited by mike; 11th August 2009 at 11:52 PM.