ah! not skid holes, edge angle holes, entirely different kettlea fish that doesn't say anything about the skids, and don't necessarily hafta line up with them because there's no skid on the edges .... most relevent is Chevys have 4 cross sills [+ the front and back edges], if you're adding extra cross sills then you'll need a custom hole pattern regardless, the position of the skid holes is where the cross sills are, so bolt spacing definitely matters - can't understand how you'll add extra cross sills without drilling more holes than anything you'll get premade
and the edge holes are related to the fenders only if it's a fleetside, you haven't indicated which it is, fleets and steps are different along the edge - if you're sure it started as a 56 longbed, the best you can do is get the skids for that bed, the holes will line up with the angle holes where the sills belong
Bill
