I did a Geodesic Aerolite, where you mount the keel AND the stringers (all held in place with forms) first. You pass the ends of the steamy rib blanks under the boat to your buddy, the two of you bow them upwards into place, and start clamping. I think the idea is that the stringerts provide a guide so the ribs come out fair. The downside is its difficult to get the forms out afterwards.

If your design has mortised gunnels, you could wire them to the forms for the rib fitting, use them to mark the (overhanging) ribs, and pop them off to do the trimming.