Look at the bottom of page 77 these pages are PDF and will print actual size. Mr. G's Enterprises - Auto Fasteners Looks like the right one.

 

That says 5/16-18 instead of 5/16-24. Could possibly work but the bolt would have to match the thread pitch.

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I'd be fine with switching to 5/16-18 bolts if the nuts are cheaper. This isn't a 100% concourse build.

 

I was in need of these caged nuts. I happen to have finished a mustang restoration and had a new unopened bag of these caged nuts i did not use. They are 5/16 x 18, which i guess is incorrect, but fit perfectly.

#64-14324, Fender to Cowl Nut Set 64-66 mustang. They are scott drake. I purchased them from Macs Antique Auto 359613-ST also a number M2K-744-G

 

https://www.summitracing.com/parts/sdk-359613-sk
Summit Racing