Just got back from the boat. My measurements, although not exact look more like 9" to me instead of 8.5. The width is 5" and that's a more accurate measuremnt..
My measurement includes the track for the sail slides. My boat is an 84. standard rig.
If it matters, I can try again with calipers or something and do a better job with the measurements... I expect to be back to the boat this weekend...
I took some pictures to clarify/confuse the issue.
The boat came stock with the spruce spreaders. Mine had been replaced by the P.O.
My mainsheet/boomvang setup seems to work well for me. I'm no marine architect, but I think it's plenty strong.