Exact wording is:
"If it sits, it fits.": Vehicles with the Fly keyword may only be placed on a level above the ground floor of a ruin if the entire base fits: meaning the entire base, with no part of it off the ruin, can be legally placed. In the case of models that do not measure distance to and from the base, but use the hull instead, the entire hull of the vehicle must fit.”
Cheers,
Thoth