As far as I know, there are no kit sources for those caps. I have a set for my 1968 Road Runner from Auto Atomix, which has since gone out of business. They were turned aluminum, which were great looking, and very accurate. I plan to scribe the "Frog Legs" emblem with an x-act-o knife, like what I did on the wheel covers on my '68 Barracuda Formula S project,
which by the way, is being rethought after seeing your exquisite project. They also did the round side marker lights, that were unique to the '68 Mopar. I wish that they were still in business, as thay had some nice products.