5/8" heater hose and can only find 1/2" fitting

My '67 Dart has a heater core with 5/8" tubes. The only fittings I can find to fit the intake are made for 1/2" hoses. Anyone else have this problem, are there 5/8" fittings out there? I have both a Six Pack and Edelbrock RPM and they both have the same size pipe fittng in them.