Get some 3" or so bolts, dimple the top of the cross sill where the bolt would go through, put the bolt through and tack the head. Make some spacer/washers out of 1/8 x 1 1/2 flat stock. Put the washer on the bolt from the bottom, center the bolt in the lower hole which is way too big and way too thin to support the bed and weld the washer to the bottom of the sill. Weld the bolt to the washer from the inside of the sill so the mounting surface stays flat.(one side is all you can get) Then weld the bolt head and dimple on the top of the sill and grind flat so the wood will sit right. There should be enough thread for the stud to fit through the bed mounting gasket and frame and be secured with a nut and washer from the bottom. IMO, six of these are more than strong enough to hold the bed in place. I haven't found a down side to this yet but I did put my bed on as a unit and my truck is more of a toy than a work truck. It won't make any trips to the dump.
I don't have a pic.so I hope you can decypher my instructions.