Whatever you decide on anodes from the protection point of view and like Vic i question the need if the boat does not have it at present - don't fill that gap in front of the prop. It is there for a purpose, primarily to let water flow in and out of the cutless bearing and, if your engine is on flexible mounts to allow min +/- 3mm (total 6mm) to cope with fore and aft movement when you engage gear.
On the anode side, a properly bonded external anode - the teardrop shape as suggested by Vic is the most common solution.