Even though the small bolts are listed as "Plain finish" -
That is not true. The only information about engine hardware available to us (remember, engines were sent to the assembly line as a "completed part", aka sub-assembly) is from Ford Car Parts, aka MPC, and as such, many were identified by a Ford Locator Code along with the Ford hardware part number. That Ford hardware part number did not include a finish number. The "Locator Code" and its associated documentation included the finish code for the parts to be made. These parts were after assembly line installation and into dealer service parts realm. If more than one finish for the same hardware part number was used over the years, that finish was consolidated into one. It is more economical to keep one item on the shelf than two, and as we've seen, Ford was "economical".
- I think a quick plate of zinc would help keep the rust away. They get painted anyway, but you can't be sure paint will get to the oil pan side.
Most of those screws/bolts were either cad plated or clear zinc plated just "to keep the rust away".
Jim