The visible pieces are mostly solid pine, but the hidden pieces seem to be glued-up panels of smaller strips of pine.
Step 1: Mod
I thought the best way to handle that was to find the center of the panel, and measure 3.5" out from that point on each side. Then I could re-join the halves back together and the pattern would match up almost like before.
Step 2: Cutting
For the seat back, I set my fence to 3.5" less than half the length of the panel. I made one cut, then turned the board around and ran it through on the other side. I intentionally left more material on the end pieces so I could fine tune the fit.