never had an issues for the 2+ years i had a heater core. seals to the neck and makes the funnel itself part of the system. the funnel will be plenty higher than the heater core. attach the funnel to the filler neck, fill the system leaving about half of the funnel full. let the car run and work the air out like normal. make sure the t-stat opens it usually takes a while just idling. once you get no more bubbles out shut the car off and let it fully cool. pull off the funnel, throw the filler neck cap on and your good to go.
funnel info is: Lisle 24610 Spill-Free Funnel, got mine from NAPA