Bed- bolt size question
 

I'm putting a short fleetside bed together from scratch-aftermarket sheet metal. Are three lower bolt on front panel to bedside 5/16 or 3/8? 3/8 will fit, but I've got a feeling that they should be 5/16 to allow a little bit of adjustment. Probably will have a couple more ???s before I'm done. Any tips or tricks for assembling a bed like this would be appreciated. Thanks in advance, Danny