jcahill:
Years ago, there were tests done on collar beams and their locations. It was found that they are not in tension (preventing rafter spread) until they are within 1' above the plate. Any location in elevation above that they are in compression, preventing the rafters from sagging from the weight of the roof, live load and dead load.
You may need to have your design reviewed by an engineer who is familiar with timber framing and have your beams/timbers correctly sized for the loads and spacing.
Good luck with your project.
Jim Rogers
