Pats58:
We believe the closest standard bolt to your original is called a "step bolt". The reprint GM assembly manual we sell shows it to be a regular carriage bolt, but originals we have checked are a step bolt. It is like a carriage bolt but the head is larger diameter, 13/16", and not as tall as a regular step bolt. You may have trouble finding it in local hardware stores but it is worth a try. 1/4" thread diameter and 5/8 or 3/4 long with square shoulder under the head. We stock them, part number 100317.
Mar-Ktech
