Rebuilding heater box, replace core?

So a little confused. The part number in the 63 parts book for the heater core is 2277535. Found it online, but everyone is saying it's for a 67-69 car. Plus nobody has it in stock. Classic says coming soon.
2277535 - 1967-69 Mopar A-Body - Copper/Brass Heater Core (7-3/4

This cross references to Spectra Premium 94503, that amazon has in stock. Tempted to try it. Why would the 63 parts book list this mopar number but all the sellers say it won't fit a 63 car? Has anyone pulled a core from a 63-66 that can verify the dimensions of this core look correct?