How about mating your body and pan this way?

[Tony Z]

I think the stiffening you desire would prob be better accomplished by installing a full roll cage and bolting / welding it to the floorpan. The body is bolted to the floorpan useing more than one bolt. It would form a fairly rigid structure itself.

A seperate rolling chassis (like the bug or older cars) is not really found that much anymore, got something to do with the manufacturing plants and ease of assembly I think, cant remember, dont care.