Thanks for the input.
I left the 12" at the bottom and made the support area ~9’ wide to allow for a skirt on the sides and bottom. I’m also thinking of eventually doing something like this: New "Clamping Skirt" Frame: 100% coverage of stock (including corners) for any width, height, and thickness of stock
I did plan for the ability to adjust the top beam in/out to compensate for stock thickness, but I did NOT think to account for the supports interfering with the chains. That definitely would be a problem with my current design. My first thought is to attach my beam to the bottom of the supports and/or to attach the motors to the bottom of the beam. I’ll have to think about that one some more.