Notice:
The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
Many thanks to all for your advice and suggestions. Here's the way it played out...
As the pan head presented a viable opportunity, my first step was to try and remove the screws with a vice-grip. There was a good hold, but the screws each sheered off.
Next step was to drill out the shank, which went reasonably well. Use a good bit...
In stead of drilling clean holes and trying to tap a threaded hole into the cast casting, I opted to clean the holes to 9/32 and ran a 1/4 x 3-1/4 bolt clear through, and set it w/ a lock nut.
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.