The bolts are 8/32, which means no drilling of the holes. I'm going to start by taking 5 disks out which will bring it back to 9 like you calculated (8.7). I'm pretty sure the 3inch series all have the ridge but I could be wrong. I've never seen them with out. They are 2inch exhaust springs that I bought out a small motorcycle shaop of them today. I don't think there hard to get a hold of but he gave me a deal on 20 of them so, I took it. The stack is of brand new Bonnville rear fenders from whats looks like from 1964 to 1970. I was pretty sure that you were going to ask me about the closed end cap.
Aaron
Aaron