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What exactly is your intended purpose for the winch? For an offroad purpose, a lot of guys use a winch in a cradle that slides into their hitch receiver for the rear end as a less permanent solution. If you mean that you physically want to bolt it into the bed, you're going to have to do some fabricating to reinforce the box from ripping apart like a tin can.
Think about how 5th wheel hitches are mounted and you'll probably need to make it resemble that, with brackets that go down to the frame. Like Davey said, I wouldn't try to mount just to the bed sheet metal.