There's also a useful section and links on this page in our wiki:
Towards the bottom, there is a link to this informative article about Canadian copyright:
I'm sorry, Bobcdy: I should have sent you a message to explain.
My reasoning was (as you correctly surmised) that one of the authors died in 1975. Canada is a life+50 years copyright country; therefore MR could not host the work on our Canadian server.
However, Project Gutenberg (your source) is meticulous in checking copyrights and follows US copyright law. If they can host the stories, then there should be no problem if we store them on our US server. It looks as though the US copyrights were not renewed.