Is this a house you own and plan to remain in for several years?
If you've got interior flooding in that room, you might just want to pull the carpet there and be done with it. Even with proper airflow, if the pad underneath is getting saturated, you've probably got mold, just from the length of time it takes to dry each time it floods.
I recommend you pull the carpet and pad, and lay down a tile of whatever style you like (pre-cut to allow floor bolts for the safe). Then you get some small, 2x2x1 inch (or so) steel blocks, and set them around the perimeter of the safe, maybe every 12" to 18" apart, so it's just slightly above the floor. That'll give sufficient airflow to allow the tile to dry quickly after flooding, but will also keep the safe low enough that accessing the mounting bolts is extremely difficult.
That's how I'd do it if it was my own house, except for the obvious point of actually fixing the flood problem.
What are you doing about the water getting on the sheetrock behind the safe every time it gets wet? That will mold too, even if you don't see it on the inside.