coolant change in 550

I posted several times the issue of how to change the coolant in 550. I received comments just open the radiator valve and the coolant will drain out. Well but what you do with the rest of the coolant in the engine blocks, which was my questions where are the plugs on the side of the engine for drainage. The ferrari has unique hosing system, the radiator hoses both comes out from the top and as a general rule of physics the water do not flow uphill unless there is a presure behind it. Finally I drained the radiator after runing the engine hot and turning the heater on, I drained only one and half gallon and the rest which is three and half is sitting inside of the engine and hoses. ( total system capacity 5 gallons).
In order to replace the whole coolant either should remove the engine drain plugs or run fresh 50/50 antifreez through it. I closed the radiator to expansion tank hose and then adding fresh 50/50 antifreez to the expansion tank. Initially hot coolant was comming out of radiator ( the coolant left in the engine) After adding 7 gallons started getting colder one( the one I was adding). I assumed with this technique I managed to change the whole coolant without getting to the engine drains plugs. Then close the radiator plug and add one and half gallon 50/50 to the expansion tank and bleed the air and you are done.