J=7 No idea
Z=7 No idea
Maybe one is for the power top or power steering? I have a couple of other tags in my notes with power tops, but the code is either L=6 or N=0. The 273 2bbl might not be on the fender tag, since L in the BDY code already means V8, and the 273 2bbl was the base (and only) V8 offered in a Dart 1964.
I think the 4th letter of the paint code is the molding insert color, so white.