Marked and drilled #40 the holes in the main rib webs to mount the new flanges. Test fit per the plans and found interference with a couple of rivets shop heads. Removed the solid rivets and replaced with Cherrymax (shop head is smaller). This fixed the problem. Clamped and drilled the new flange to the spar per the plans. Removed the ribs and drilled the #40 holes up to #30.
Had a similar problem with the nose rib flange. Talked to Kevin who suggested using round stock to create some small clearance "dents" in the flage to clear the rivets. I used different diameter bolts and the round shaft of a punch to make the dents. This worked and after much trial and error, I was able to get the flanges to sit flush. .