On a different note, high-wing aircraft gain some roll stability from the CG being well below the wing. Which acts as a pendulum that tends to "right" the aircraft if temporarily disturbed - e.g. by turbulence. Rather like a lead-filled keel in a sailboat.
Which allows for less need for wing dihedral for stability - you will note a high-winged C172 needs/has less dihedral than a Piper Cherokee with the CG above the wing.
The Beverley is like an overgrown Cessna, high wing with no dihedral in the center wing section, but more in the outer wing sections - net, not a lot. Therefore its design counts on a low CG for stability.
The payload space in the tailboom increases the chance of getting a high CG, if that "attic" space is filled with troops like old,notbold. Thus care must be taken.
As to the Super Guppy, again a comparison to the much smaller Piper, which has "just enough" dihedral for normal loading focused near its CG. If the Piper was given an extra-tall cabin to replicate the Super Guppy layout - without a corresponding increase in stabilizing dihedral, it would be (in another nautical analogy) like standing up in a small boat. CG rises, and boat get more unstable in roll (tips over).