Using fluting pliers to align rivet holes
My suggestion is to first get the rib flange rivet holes aligned with the use of "fluting" pliers. Then gently adjust the flanges to 90 degrees. The fluting pliers crimp the flanges which allow the individual flanges to curve. I don't have a flexible metal straight-edge to check alignment with. I use a reasonably straight strip of stable white oak maybe 1/16" thick by 3/4" by 20" that bends easily. I'm satisfied if all the rib flange holes line up within half their diameter--about .050." Then I gently adjust the flanges to approximately 90 degrees. With the flanges creased/fluted, they seem to be flexible enough to adjust the last little bit if needed when being attach to the skin.
Just my two cents. I've gotten the empennage kit completed and working on the wing outboard leading edges. I'm pleased with the wing's top skin attachment between the main and rear spars. Good luck. Oh by the way, when adjusting ribs as described, it seems the ribs naturally lay flat on a surface which I really don't worry about. Hope this helps.