I couldn't comment whether the plane is defuelled before it goes in the hangar - I would have thought that it would have some fuel in it but no passengers/cargo. So I would design for partly loaded, and include the heavier 737 models.
Slab thicknesses (subject to assumptions and not to be used for real) for the 737-500 empty at 32 tonnes needs 210mm, and laden at 61 tonnes needs 325mm. This is on a fair-poor subgrade with a 150mm cement stabilised base underneath, and no frost.
The 737-800 needs 345mm laden at 79 tonnes and 230mm unladen at 41 tonnes.
Seems to me that a concrete floor slab 300mm thick would keep you out of trouble for years to come; and 250mm would hold up for a few years by which time everyone will have forgotten who built it. I'd choose 300mm because concrete is hard to fix once it breaks.