Dimpling, too!
The problem of reaching in to those narrow places doesn't just happen when riveting. It's hard to dimple those holes, too. I found that I didn't have anything that would allow me to get my dimple dies in there and do the dimpling. Even a pop-rivet dimpler won't work because it's so narrow in there, you can't get the nail in there and through the hole, and out again when you're finished. So I got a little creative with this, and I'll share my method here with anyone who is interested.
Before I started my project I bought a set of bucking bars that included the one seen in this first picture. It came in very handy throughout the build. This is the tool set-up that I used for this job. Besides the bucking bar I mentioned, you can see my rivet gun, my pop-rivet dimple die set, and a few holes out at the end of the rib that need dimpling.
Next, I cut a nail short, so I could get it in there and through the die and through the hole in the rib. If you get the length of the cut-off nail just right, it will be long enough to hold the dies together, but not long enough to protrude through the other die on the outside of the rib. You'll see below why this is important:
Here, you can see the concave die inserted in place inside the rib. The short nail is through the hole:
Then, you put the other die in place, and use the rivet gun and bucking bar to hammer a nice dimple:
All finished! This actually worked out very well. Just to make sure I had a good dimple, I used my countersink cutter and cage to touch up these holes. That probably wasn't necessary. I observed that the cutter removed only a very tiny amount of material.