Amount of coolant in the cooling system depends on motor and radiator. A 273 with an HD radiator will hold 22 quarts. A 273 with a normal radiator is about 18 quarts. That covers coolant in the radiator and engine block. As you know there are 4 quarts in a gallon. I like Prestone myself and always buy the stuff you have to mix. Don't waste money on the premixed stuff.
When I flush mine, I remove the block drains and drain the block also. You typically have to punch a hole in the drain to get it draining.
Once its all back together, I'll put a gallon (4 qts) of coolant in the radiator, then a gallon of water, then pour half of the other gallon of coolant into the empty coolant container. then fill both the rest of the day with water. Fill the radiator, put the cap on, run it until it gets to normal operating temperature, let it cool, check level and fill the rest of the way.
That's how I do it.