Hi Mike,
The spreader bar just helps the top of the frames stay in alignment, it is made from scrap wood and the piece I used just happened to be the same thickness of the full frame. It does not have to be that thick, it could be the same thickness as one side of the frame and do the job.
I am glade to here you are ready to start the frames, make sure you download the latest copy of the frames I stubbed my toe on the first batch.
I am working on Part 3 and you will be working from proven drawings for this part.
Mike