To clarify: I have never seen "Japs", or "John Jap", or "Nips" as racial slurs. It is the equivalent of "Tommy/Tommies" or "Jerries".
"Japs" is short for Japanese; " Nips" is short for "Nipponese" (which is closer to the Western version of pronouncing 'Nihon' =Japan).
EDIT: there are instances, where names used 70 years ago may now be inappropriate. Although, in an historical context, how far do you extend censorship?
e.g. the name of Guy Gibson's dog.
IIRC, based on a Japanese friend I have, he said that Nips and Japs were racials slurs then and now. I'm not sure if it's really true but I would stay in the safe side and limit on using it. Then again, we have soldiers calling each other Ivan, Kraut, Rus etc etc.
