Originally Posted by P.C.

Just remember that if you fill the system with only water and cannot drain all of it out, you have to add antifreeze in a high enough concentration to ensure that the entire system winds up being filled with 50% antifreeze/50% water. My father once filled up his '68 Cadillac cooling system with roughly 90% water, then couldn't figure out why a cold snap resulted in the engine block's freeze plugs popping loose because of ice forming in the block. It was a miracle that he didn't crack the block.

Originally Posted by Zulfiqar

there are 4 ports in the lower manifold that channel coolant from both heads. You have probably gone down on level quite a lot but if you are unable to find the block drains then get a leaf blower and make a setup that you blow air through the heater hose that connects to the lower intake manifold on the side of the thermostat housing, Open the radiator drain or remove a hose and get a huge pan under the car to collect the coolant that shoots out. Measure all of it.

coolant does spill into the engine and you have to be very careful that it does not flood the engine - thats why I recommended 4 strips of blue shop towels pushed into the coolant ports of the head and hang them outwards from the engine, they wick the coolant and drain it out rather than into the cam valley.

I see that you are 90% done removing the pretty bits from the engine to see the lower manifold, you are only short on removing the upper hose and the heater metal hoses from the manifold. after you take off everything including the chintzy 4 bolts you may require herculean strength to remove the manifold if the previous mechanic tried to be goofy with it and applied some adhesive.