For someone doing a coolant swap, do you GM Techs NOT recommend draining the block from the two drain plugs underneath? It's not in the GM manual, and I am wondering why it's not in their manual, because wouldn't you want to try draining as much as you can before refilling with new coolant? And what do you think about in addtion to refilling via the reservoir per the manual, but also filling the engine block through the hose that leads into the radiator inlet tank, especially if I drained the block through the plugs? THis would be for 2003 Silverado 1500 HD, CC. Thank You!