This is the area that almost all of the horsepower (and there can be A LOT) is transmitted. Further to Strykers advice, mounting the mast in a tube/box/receptacle which is a structural part of the fin box is good. All the forces transmitted down the mast are transferred directly to the fin, and the hull becomes less stressed and more a device for directing the water past the works.
Any structural bracing belongs oh the inside - you dont want anything that is going to disrupt the water flow on the outside. You do not intend racing, but looking at what practices the racing lads go in for is a good idea, as they want a boat that will perform well in a wide range of conditions, and this is probably what you want as well.