Interesting - thanks - I was wondering how that could be done. If you come up with a source, please let me know.
Looking at the adaptors you posted - obviously they're a permanent weld on thing, and they don't seem to have what I would feel is the required compression bump and under-seat that would prevent the cap from working its way OFF the nubs once installed. In the original installation on the wheel, the cap has to stretch a tad to go over the bumps, and then relaxes as the cap's rim goes past and under the nub present on the factory rims. That way they're very likely to stay in place. The adaptors are simply compression with no underseat area, and I would think they might permit flex movement that would ultimately result in the cap leaving the wheel.
I was thinking of a spring ring that went inside the larger hub cap and provided the grip around the wheel's hub nubs. It would have to have a gap in it to allow it to expand and contract on installation as described above while still holding on to the oversized cap.
I also like these caps which are 9", I believe, but fek, they're expensive.
Wheel Center Caps
I would like to use the proper 66/67 Dodge doggies, but I'd like to get some sort of adaptor ring that allows me to use the wheels in other situations.
@Mike66Chryslers - how did you mount your lovely Chrysler ones?