I reckon you should be fine with the 40cm, the Futura has a pretty wide tail and anything smaller than a 40cm might be prone to spin-out. But also depends on which Drake fin you have. Your 5.5m may well be better on a 430 c/w a 460. If the 460 is not the recommended mast (to stiff) the sail will feel switchy, a softer mast will make it feel softer and easier. The sail should have a label showing the recommended mast or IMCS (IMCS 21 = 340 mast and IMCS 25 = 460 mast). Most 5.5m would use a 430 mast.
Cheers