From what I've read online, many of the nikons didn't need heavy modifications, some had the type of lubrication and grease changed, as well as the glues on the coverings. The rest was attaching motordrives and making the camera easier to aim and shoot with their gear on. The vast majority of nasa cameras never left the earth, and were probably used for documentation on the ground though and wouldn't have needed these mods.
I remember seeing a documentary with old footage of a few of the moon landing astronauts learning to use their modified blads in the dessert practicing with focusing etc. that was pretty cool. Too bad there isn't a book on camera gear that was used specifically for space or near space, I would love to have something like that. I would also be curious to how they tested and evaluated cameras and how the bidding process went with each company, and why they ultimately chose what they did.