Rebuilding heater box, replace core?

When rebuilding my 1964 & 65 heater boxes in parallel, plus a spare 1964, I flushed and tested all the heater cores w/ air in the swimming pool to verify no leaks (used a bicycle pump to 20 psig). I recall re-using them even though I had a new core which only cost ~$30 (Canadian, forgot brand) since no leaks and no green showing. If a factory AC car, I think the heater core is a different PN, and in 1963 AC was quite rare, so might need to be rebuilt at a rad shop. I use Evans Waterless Coolant so no fears of corrosion or need to ever flush it.