Interesting idea to raise the seat - perhaps a solid spacer with the longer bolts. I would like to use that space too.
However, my concern would be safety in an accident - the shear load on the longer bolts now has a new tension aspect. I just wonder if the ability of the floor pan thread can handle a higher tension load, which combined with shear increases the concentric force?
Perhaps some of the engineers out there can comment - maybe I'm getting my definitions mixed up and I'm not using correct terms...
But my initial thoughts are that raising a seat up weakens the structure...