Remove all water from the block

And the 50% mix is good to -35C as I recall. That would be -31F, if my math serves me.

I know nothing about BBMs but on SBMs the 50% can barely be reached by draining the system through the rad petcock and also draining the heater core.The 26 inch rad makes it a little easier.
You know and I could never figure that out, cuz it seems like the block could never hold that much water! I usually remove the one waterpump bolt that also penetrates into the jacket. Drivers side, 2nd from the top IIRC.Gives me a chance to refresh the anti-seize on it. But it's always better to drain too much.