Joined: Sep 2012
Location: Sydney Australia
If you have an original style copper core, then stick with that.
If it looks good and pressure tests OK, then you should be fine to use the one that is already there. If there is evidence of corrosion, then replace.
Just make sure you buy a heater box seal kit and get the foam rubber heater core end caps. These help protect the core from vibrations and damage and make the core work more efficiently.
15/5/1964 260 Manual Coupe.
23/3/1965 289 Manual Fastback.