Sometimes you just have to find a way to get it done. Maybe that means grinding down the tip of a tool, bending some aluminum out of the way, grow a third arm or even seven fingers. To this day I remember the time it took me 5 hours to put a single SAE nut onto the aux alternator. And I wrote about the agony in my http://www.rv7-a.com blog so I wouldn’t forget.
Getting the metal countersunk for nutplates in this limited access area wasn’t so bad though. Just take 1/8 inch off the length of a 1 penny nail and use the pop rivet dimpler. Hope you have a longish no-hole squeezer too.