Your false floor will be 9-10" above the current one. But you can pick up some storage around the tire beneath the false floor... Be advised you don't want to contact the frame crossmember with your tire well without isolating it somehow, unless you like buzzes and such. Also, the stock tiedowns are not sufficient for holding a tire - take them to the frame (again, rubber/urethane isolation from the body is recommended).

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell