I’d think a blimp or some sort of lighter-than-air craft would make a better testbed for first solar trans-oceanic flight. That would take some of the clinch out of nights on battery power alone.
I think they wanted to do it with a plane providing the lift. A gas bag would be larger and might get into the size and power required to move it problem. Someone already went around the world nonstop in a balloon.
First nonstop across the Pacific was in 1931. Japan to Wenatchee WA.
Took 41 hours. Single engine Bellanca. Two pilots. Plane was rated for 600 gal of gas and carried 900. Landing gear was dropped on takeoff so chief pilot landed it on the belly.