Thank you for your comments and suggested solutions.  This problem was back on my radar screen from a posting here on AgTalk a week or so ago -- and I was actually asking my machinist earlier today as to which rivet-clenching tool he thought would be best -- a big set of pincher type with long handles, or the screw-down type that you have kindly photographed and posted.  We were thinking of either offering them for sale, or put down a deposit and we loan it to you until you're done with the install. 

The fasteners as well as the clenching method were borrowed from RK Products, and I simply hadn't revisited the tried and true, although I admit those rivets aren't exactly easy to install.  As to using a bolt there (front hole), even if it's fine-threaded, with lock-tite, that doesn't work for most people -- there's not enough remaining to hold it together after you grind it down so it doesn't rub.  How much clearance you have to work with in that spot depends on opener blade flatness, and how tightly they're shimmed.  There really isn't room for anything thicker than a squashed rivet.

Thanks for all the suggestions, guys.  I think the best method is to squash the rivet as much as possible with the threaded crush-clamp (such as is used for sickle rivets), then give it a good wack with a hammer at the end to make it truly solid by swelling the rivet.