IIRC, Motor City cast them a long time ago. I've seen them cast in the past, and even may have had a set at one time. But it is hard to get them looking just right, takes some mods with the wheel centers.
Recalling the era, most guys got the bottlecaps because the first trip from home was to Beck's Wheel City for a set of Ansen slots or Cragar S/S, with the stock wheel tire combo relegated to the back of the garage.
The next-best option to a Magnum 500 would be the '69-'70 Shelby GT 500 wheel. There was once a black '68 GT shod with 4 of these, and small letter Goodyears that was gorgeous. The current Revell GT500 has a decent set; the '68 Pace Car Convert and '69 Cobras both had them on the tree.
There were also a few wheels that Ford designers would frequently use for their presentations to jazz up the clay models - Kelsey Hayes "Magstar" and the rarer Motor Wheel "Apache." GT350 recent release in the faux '67 box has the magstars, I cast the Apaches.