Remove all water from the block

You don't need to remove all the water from the block, you just need to remove 50% of the water so you can add enough coolant to get to the optimal 50% concentration of coolant to water ratio.

Look up the total volume of coolant in your system. Drain the radiator from the petcock, if the amount drained is greater than 50% of the total cooling system you're golden. For example a 340 Dart had a stock cooling capacity of 16 quarts or 4 gallons. Thus 2 gallons or 8 quarts of coolant is the correct amount.

If you're still worried, you can test the concentration of your coolant. They sell hydrometers at auto parts stores that will tell you your concentration of coolant.

Hope this helps,

Regards,

Joe Dokes