c6joe,
I would suggest, if you haven't already, removing the covers from the pages your Grandfather has mounted them on. I know you love the way he has displayed them, but perhaps you can just lay it out the same way on new pages.
The paper your Grandfather used would have contained acid and lignin, which has caused them to discolour the way they are. This will affect the paper covers mounted on them.
For covers this significant, you really need to look after them and house them correctly, using archival storage methods. They're well worth preserving

Thanks for the scans, especially the Earhart cover. Awesome.
Balf