Our rad has the same tag as Jeff's. I took it to be an "O" with a slash through it. Whatever, they are the same and are three core. We had the rad in our 455 Lemans done with a high density core and it is more than adequate in the cooling department. Our 427 never had a problem either. Mind you we don't get 120 degree weather here either. Low 90s at the worst.
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Chevelleless after 46 years......but we did find a low mileage, six speed, silver 2005 Corvette. It will just have to do for now.