I've been reading the Stars Without Number rulebook, and wow, it makes me want to hack it to run a 40k campaign. I'd probably struggle to find players with the people I know, but that won't stop me planning it.
The Retired Adventurer has done some posts on hacking SwN for 40k:
Putting these here (in no particular order) as a reference for myself. I can't find anyone else who has done this so any links to other ideas would be appreciated.