the 1x6 you refer to is a good idea, let it into the outside faces of the rafters just under the sheathing and it will look great... framers around here call it wind bracing and install by tacking the board in place, tracing around it, then rough out the rafters with a circular saw set to match the thickness of your board. After that use a ripping hammer to clean out the cuts, a chisel is not usually neccessary.
