Susan, I think that the blocked-off heater core is very minor. If you have to turn your heater on to help cool the engine, your problem is much bigger, and elsewhere. How many cores does your radiator have? If it gets too hot after 30 minutes of highway use, I would look into the airflow through the radiator cores. For example, I purchased a 3-core Challenger radiator for my Plymouth, but what I received was a 2-core with the same part number. I called Modine, and their tech told me they've replaced their 3 cores with 2 very wide cores. Same amount of coolant, but considerably greater airflow through the radiator. Further, notice advertisements for 4-core radiators. They strongly recommend a more powerful fan. The reason is the air has a harder time passing through all those cores. Hope this helps! Sean ---- Please address private mail -- mail of interest to only one person -- directly to that person. I.e., send parts/car transactions and negotiations as well as other personal messages only to the intended recipient, not to the Clubhouse public address. This practice will protect your privacy, reduce the total volume of mail and fine tune the content signal to Mopar topic. Thanks! '62 to '65 Mopar Clubhouse Discussion Guidelines: http://www.1962to1965mopar.ornocar.org/mletiq.html.