Back in the early 70's AMT molded it in multi pieces so they could use the front of the cab for the non-sleeper version. Easy for them, difficult for builders.
I end up assembling the cab and either A). Live with the visible seam, or . fill the seam, lose the rivets and reapply them with Archer or Micromark decal rivets.
The sleeper section of the interior tub seems to have grown over the years - I leave it out until after the main interior tub is slid into place, then I put the sleeper section of the interior in (it might need additional trimming to fit easily). If you assemble the main interior and sleeper section and try to fit it in, you will probably break open the cab seams creating much frustration.