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Dump the OEM radiator garbage. You're going to fry your motor if you don't!

There's B-Cool, Griffin, Durodyne and a multitude of other custom performance radiator manufacturers who makes a radiator that'll keep your BBC cool. You'll need dual 1" or 1 1/4" tubes as Rick said. I'm running a Griffin in mine and it has dual 1 1/2" tubes. Griffin also has a geat website. You can choose radiator sizes and inlet/outlet locations to match your configuration. For a minimal fee they'll even custom make one EXACTLY like you need.