I recently pulled a head off a 318 Magnum. all I did was crack the lower radiator hose and let the system drain out. there was still a lot of water left in the block, but the coolant was low enough that I didn't fill the oil pan when the head came off.

however, the coolant was high enough that when I lifted up the front of the truck a few inches, water came out the back of the block, but it was the lower holes on the deck, and it dripped out to the floor.

after you do your head work, I would change the oil incase you do spill any water into the pan. Otherwise, have at it!