I deviated from the plans here where they tell you to countersink E 702 spar for the flush head rivets that go under the elevator horn for the E709 rib. I even had a discussion with vans technical support and they agreed that dimpling these holes would be acceptable and he actually stated that it would be a better idea to dimple instead of removing material. I will say dimpling the 709 rib would have been almost impossible without the close quarters dimple die set that you use with a rivet puller. You will have to buck these rivets because a squeezer wouldn't fit into that curve. In the end it worked out well and I believe much better than the original instructions.