I have twin 2003 fresh water cooled 8.1L Horizons and need to replace the antifreeze coolant. The service manual says that the coolant (not the seawater part) can be drain thru an hex plug located 6" behind the engine mount. I can’t find that plug. Does anyone know of a schematics/drawing that would show it? Does anyone know what size is the hex? Thanks !

Looking at the drawing it appears to be on the Port side of the block between the center 2 cylinders