While not specific to your year truck, the "How to Build Tri-Five Chevy Trucks" has detailed sections on restoring the floor, cab corners & treadplate (including the support). If you get a chance to look at it, I think you would find it useful. The book is listed in the Summit catalog (under books -page 218 in my version). Also I have seen similar articles in one of those "Classic Truck" type magazines. Another source is Tom(?)Brownwell(?) "How restore your Classic Chevy Truck." I'm pretty sure that is not the exact title, but most of the old truck parts suppliers such as Chevy Duty have it.You'll recognize it right away.