I just started playing Plutonium less than a week ago and I do think to avoid a C&D letter from Activision, you should take down any way to download game files from the Plutonium website and add links to the Steam pages of each COD game.
The actual games themselves, the DLC, all that stuff, I think distributing game files is what got Xlabs taken down sadly. I don't want the same to happen to you guys. It's been wonderful playing these older CODs and reliving old memories.
Hopefully you can combine Xlabs' into Plutonium as long as copyrighted game files aren't distributed, and continue preserving these old games.