Heres how i would do it. Drain your coolant and fill with distilled water ($0.89/gallon) and prestone radiator flush. Idle vehicle to temp and run for ten minutes with the heater on. Drain that bad boy again (just pop off the lower radiator hose). Fill with distilled water again and drive a bit. Repeat until the water doesnt come out nasty orange/brown anymore. Then drain and fill with coolant. You can buy a cheap tool at the auto parts store to test that you have the correct mixture. Ive recently done a 4.2 and a 2.5 this way, no problem.