IIRC, you can shave up to 6 thou off the factory head of a AN426AD3 rivet and still be in spec.
Use a heavy duty countersink cutter cage like the one found below:
And a rivet shaver bit:
The gear appears to be a bit more expensive than it used to be, as when I purchased it I think they were ex surplus stock.
Attach it to your favourite high speed drill and go at it, taking off a little at a time.
Alternatively, if you don't want to outlay such capital, others tape around the area and shave down slowly with a dremel or sanding block. Main think is it passes the pressure tests.