Just to update for anyone else who is also searching.

Got some 14in OEM wheels from Summit Racing: 14in x 5in / 5x4.5 bp
Went from 13s to 14s; they fit fine and now I'm sporting new tires; van drives smoothly. They came primed and I painted them glossy black.

Looking for some OEM center caps now: need 10.125 in snap-ons.
Anyone know of a good source? I've seen some generic baby moons, but not sure if they'll clear the grease cap on the front wheels; only protrudes about an inch past the snap on nubs.

Thanks for all of the help