The rivets that go into the bottom skin are AN426AD3-3.5 for the flange without the autopilot bracket.
The upper flange rivets are AN470AD4-4 without the bracket.
Adding the bracket would need a slightly longer rivet, probably a 3-4 or 3-4.5 and 4-4.5 or 4-5. depending on the thickness of the bracket.
They are cheep enough to buy several sizes. Van's sells them in 0.01 lb increments for about 46 cents.
Installing the lower rivets will require 2 people.
If you are wanting cherrymax pulled rivets, someone else will have to chime in as I am not very familiar with those.