I just did this job on an M112. I did not drain the radiator because the coolant was flushed less than a year ago.

I ended up losing 3 quarts of coolant. For purging, just park the car with the front higher than the rear, open the cap, start the engine, then squeeze the up radiator hose a few tims when the engine warms up. Realy no big deal. You could also fill it, drive for a day, and check for level.