[mr48chev]

As Newfisher said, it isn't the tires and wheels as much as it is the fact that the flatbed sits low and doesn't give much room between the bottom of the bed boards and the top of the tire.

The simple fix is to not carry loads on it outside of a cooler and a pair of lawn chairs. Since that probably isn't the plan I'd suggest either a set of overload springs that clamp to the axle and ride against the frame when you get a load on the truck, a set of leaf style overload springs, Air shocks or make some wheel tubs out of some heavy plate or diamond plate and cut out the boards over the tires and put the tubs in. They could be made low enough to not show to the side over the bottom board of the stake racks but would give you four or five more inches of tire clearance.