1988 390 EC Winterizing

[electricaldoctor]

This will be my first shot at winterizing this boat. I had hoped to simply introduce 50/50 anti-freeze into the strainers with the engines running, but the outflow from the strainers are right at the top and I am unsure if this approach will be successful. Does anyone have experience with this vintage of boat and similar power setup? (twin 454 gassers) I also have similar raw water strainers for my Gen & A.C. units.