I took the original to my very good radiator Repair shop and he will test it but he said looking at it, it looks to be leaking. He tried to get a core and his source says not available. Looking for help, thanks.
Go to a parts store. Or better yet if yours is a Harrison original get it recored. Not sure where you live but Brett has a good guy here in Mass.
Paul, If he's as good as you say, he'd know where to get a core. Maine Auto Radiator in Lewiston, ME is the place he wants to go. I'd offer to have my guy up here in MA do it but am sure the shipping would be killer. "Maine Radiator MFG." https://maineradiator.com
A heater core should fit in either the medium or large USPS flat rate boxes. Two radiator shops, left here that could possibly fix it. 1. las Cruces Radiator on W. Picacho St. 2. Lohman Radiator on Lohman Ave