I got a 01 monte ss and i Need to flush my coolant since the previous owner of did not use dex-cool and I wanted to flush the block with water but I need to know how much the block can hold so i get the coolant to water ratio right. I know the 2 knock sensors are drain plugs for the block that i can use to empty it so I am able to fill it up completely with premixed coolant but id rather not touch those at all.

 

If you go as far as draining the block by pulling the knock sensor and draining the radiator you surely can just add premixed coolant and just top off. I might suggest flushing the coolant first and back flush the heater core ( remove hoses from engine and run water thru both directions) if it's been a long time. Good luck