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These are pictures of where the spars overlap.
      In the first picture below you can get a feel for the width of the fuselage. At this point the fuselage is tapering in. Two sets of knees would be just on this side of the spar. Two butts would be just on the far side of the spar as the fuselage widens out to its maximum, about 40”. Narrow yes, but wider than a standard coach seat at the hips.

      The second picture shows the two overlapped spars from the bottom.  With those big angle pieces it is quite thick and strong.