Sometimes we have to drive rivets in blind spots like under a longeron or behind a bulkhead. This trick has helped me many times to locate a rivet in a hole, align the bucking bar, and look at the finished shop head.
Place a mirror in a stable location so the reflection you see is the work area. Use a shim to tilt it if necessary. Get creative with tape and shims to set the mirror where you get a good view.
As they say, “A picture is worth 1000 words.” I think the photo explains it well. If you look closely at the mirror in the photo, you can see the next rivet shank at the far left corner and several shop heads. Build on!