Are there any specific weight or structural reasons to choose low vs. high wings for a cargo aircraft?
@Ron Beyer has pointed you in the right direction.  Study existing designs going all the way back to the Gigant.

The wing is by far the strongest part of the aircraft as it must carry leveraged aerodynamic loads and the weight of the aircraft while flying. A low wing may be slightly stronger to positive G loads as the fuselage forms an arch over the middle of the wing.  However, for cargo transport the high wing offers the advantage of a lower set, roll through cargo bay and more stability in flight.

As far as landing gear, the low wing offers placement
of gear on wings for wider track. High wings mount theirs in the fuselage, but the high wing offers the opportunity for greater ground clearance of obstacles on "unfinished" landing strips.

As you can see both are made and both are in service worldwide.  Commercial freight would favor the 747 type, military the C-17.  And let's not forget turboprops for fuel efficiency!

Both high and low wings will fulfill the requirements for cargo hauling, what about both on one plane to get wingspan back under 150 feet!.