Heater Core replacement

Ok I can tell you that the:

Cores from 65-69 with or without AC will work fine in the 64 Dart.
PROLIANCE Part # 399104
FOUR SEASONS Part # 94503 with AC

And I think you are correct the firewall in 63 is not the same so i do not think they work for the 63's.

Finished mine today. Went with the FOUR SEASONS Part # 94503 with AC
it seems like it is built alittle bit better.