The link provided above is indeed very good. I will also add that it’s unlikely the documents contain classified information. Typically people with Top Secret clearances are very conscientious about proper handling of materials. Also, at that age, they are 99.9% likely to have been automatically declassified. So there is strong reason to default to “make available” instead of “deny access.” I’d definitely suggest talking to the NARA folks prior to opening a formal declassification review.
https://www.justice.gov/open/declassification/declassification-faq
https://www.archives.gov/declassification/iscap/auto-declass-exemptions.html
And you’re quite right, FOIA only applies to federal agencies. If someone wants to FOIA this information, they should file with the appropriate branch of service. On the other hand, if it could be retreived via FOIA (or even just an archives search), why not make it public and spare your requestor a lot of hassle?
https://www.foia.gov/faq.html