A "better" Autoparts store (the type that serves professional repair shops,) and even some of the larger chain autoparts stores should be able to suggest the next colder range plug.
I would have to say, that I spent some time at the Autolite website, and though there is good general information on spark-plugs and heat-range, even plug cross-reference charts, but it was not "easy" or instantly clear as to the exact plug that would be the next step colder.
Autolite Tech Section I suppose, if you went to another manufacturer (say, Champion) and cross-referenced an Autolite 85 plug (a D7) with their competitor's replacement plug, then figured out the next colder plug there, you could cross-reference that plug back to the correct Autolite.
Or, you could EMail Autolite and ask them directly what their next colder plug would be.
Or, you could switch to another brand of plug.