Hawkman - In case your still hunting for the part, below is a screenshot form a 1955-59 Chevy Truck FAM with the item you need circled. The 1957 Chevy Master Parts Book lists it as Group# 4.634, GM# 3712797 - Retainer, brake pedal shaft (1 hole 11/32 dia.), which is the retaining bolt hole for the plate.
Using the diagram and measurements from your setup, perhaps you can fabricate a replacement from some flat stock?
Hope this might be of some help.
