I went with white. There are several places that have gray, Bike World USA for one. I thought white looked better with the violet paint, besides, that's what was on it when I got it.
The original, (rotted) Schwinn tires were gum wall, so that's what I went with.
I've lost track of the site that has pictures of them in the catalog, but the picture I saved from it looks like white cables, and it definitely has black wall tires.
If you don't know about the Schwinn forums, here is a link:
Collector's Forum - Schwinn Forums
Great source of knowledge about these old bikes.