Have you considered doing the lapping before the frame is raised, assemble it flat, squaring and leveling the wall systems and lapping the braces?
I don't use the type of joint at the corner sill you described, but I suspect it will still allow a little play for the bents to be tilted enough for the wall girts to be assembled.
Here is a rough example of heavy chamfering, this allows the joint to be started with easy. As apposed to a little blunt 45 degree cut on the corners. You can even ease one end more to help allow a member to be tilted into place such as the wall girts,
http://www.youtube.com/watch?v=92SgPPLgyXA
