The front panel to front cross-sill and front panel to bedside bolts were 1/4". Since there are quite a few of them at each joint, 1/4" bolts are plenty strong enough.
The rear cross-sill to bedside bolts were 5/16. Since there are only 4 of them per corner at the bottom of the stake pockets, GM upsized them a bit.Mar-K's catalog shows them as indented hex head bolts with an integral washer on the head for the rear, if you want to stay with the original style. Their catalog # for zinc plated bolts is 100577 for your truck if you don't already have them.
