So it seems that 70 years is the limit, which of course includes
most or all of the material we're interested in.
While still intending to actively seek permission, i wonder whether
a registration system might escape the legal bind. The site would
not be 'public', but interested scholars, etc., would register, and
as part of that registration would state that they were only using
the material for their own study. Perhaps there could be a built-in
limit that each user could download the material once only. I don't
know if this would help the situation or not.