Make a big fat reinforcing plate for each ring beneath the floor and it should be just fine. Use fatter rings if you want more.

Tires are extremely heavy, hence the risk. That's why I won't run one on the roof; exactly the wrong place to put a lot of weight. For normal cargo I would not worry. Still, strapping to the rollbar is always a good idea.

-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