We gotta a thing at work we use to refill coolant, it pulls a vaccum down on the system and uses atmospheric pressure to push the coolant in. Never have to top the system up after warm up, and no air lock anywhere in the system. It uses a small venturie and your shop air compressor to pull down a vaccum. Best thing you ever buy and use it on any vehicle, $120 or so I think. As for draining the coolant I would assume there is some kind of drain cock on the bottom corner of the rad like all other GM's I worked on are.
2005 GMC 2500 HD, LLY, Ext cab, 4X4
EGR Block plate
Edge Evo Cs