As for tearing apart a 35mm I would not suggest you do that. I took a course at what used to be a school in Colorado called National Camera Repair School and did a little repair work later. The main problem you will have is you might not find all the springs and ball bearings that fall out of the things, and that you will throw off the timing of the shutter. With out some form of testing device, you won't have a working camera that is accurate. As for replacing the foam in the RB67, I and some others use felt instead of foam. I buy little sheets from Wal-Mart and cut them up and glue them in. You cut them about 1/16in. (or 1-2mm) thick. Felt doesn't become a gooey mess like foam does. Not everybody recommends it, but it is a cheap option. For the mirror, I would suggest staying with foam. If you are an older guy like me, that big viewfinder is a joy to behold. As the eyes go south, looking into a little window of a 35mm is not like that big TV screen that sits on top of a RB. (Even if it is backwards.) Ric.
P.S. If you are serious about camera repair, there is a course by a company called something like CC Camera Repair Course. It is on the internet if you do a search for it.