The location of the drill holes turns out to be more critical than I thought. I tried to match the existing holes in the shovel, but apparently I must have slightly flexed either the shovel or ruler while measuring. This caused the end result to be slightly uneven, instead of a precision edge. This did not cause the shovel to fail, just that the wearing caused some areas of the ruler to disappear before the others.

I used a work bench vise to hammer the old rivets down.  Thought about using easier to get and use bolts, but they stuck out to much. Rivets work better and are easily replaceable if (when) they wear out.