I've broken studs off with them so I think they are as strong as they need to be. But to answer the question, the correct long chrome nuts for the "bolt-thru" wheel covers were for many years exclusively sold by Frank Mitchell for a fairly steep price ($125 for 20). I believe Frank is gone but they were still available on eBay a few years ago. But I just checked eBay and I couldn't find them. The generic ones you can buy new are shorter and usually have the wide base, which technically is probably a good idea but isn't correct. Here is an original LH and two typical shorter nuts. You are looking for 7/16-20 for an A-body.
Ah, while I was writing, I see Alan has found the link...
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