Sounds like a perfect application for square rule joinery. Jack Sobon's two books on timber framing provide the most coherent and user friendly explanation of the process that I have seen. The basic idea is to work to an imagimary square timber (say 7.5" x 7.5") inside your bowed 8x8. The timber is reduced as needed to 7.5x7.5 in the area of the joints and the joinery is then cut. You calso end up working off of a snapped chalk line to establish "straight."