Perhaps not the answer, but the instructor in my diesel school
said never to change from one type antifreeze
to another, i.e. never go from green to yellow as it may turn to jelly and be extremely difficult to remove.
Alternatively, my westerbeke
has a port (pet cock) on the inferior aspect to which a hose could be attached. Yet again with a threaded tube the size of the zinc a hose could be attached. And the thermostat could be temporarily removed. Mine is well have the coolant manifold. .