How rusted through are the bottoms? Three things come to mind:
The first was a infomerical on TV, they had a spray can of stop leak, if you ordered in the next five minutes you got double the order. Just pay seperate shipping and handling.
Second idea would be if it isn't too rusted through, maybe you could put a layer of foil over the rusted area and then spray it with a nice thick coating of bed liner.
Third idea, fix it with fiberglass. Or maybe auto body putty.