under a right side plate
As you are looking at the SB8, there is a plate on the right side of the block with a stud and a nut mounted on it at dead center. The stud is a part of the zinc, but take the plate off first. You'll need a rubber washer and a gasket in addition to the zinc. www.dieselmarinedirect.com sells these parts. If you find the zinc supercorroded and the block full of crap, you might consider removing the head and the exhaust manifold to make sure that the water jacket isn't gunked up to the point of localized overheating.