Pedantic I know, but the Airbus ‘STA’ aircraft are not Guppies - they’re Belugas. The Guppy aircraft was based on the Boring 337 Stratocruisers and haven’t flown for decades. Beluga is based on the A300/A330 aircraft.
In this instance I doubt their “volume” would be much use as their bulbous bodies allow for additional volume “up top” and “out-wide” but the floor itself is only standard airliner size. You can’t stack a pallet on another pallet so little benefit.