that is a puzzle. I found some of my missing family of working age living with other families being a farm hand and apprentices. He may have been moving during the census year and not been listed anywhere. Hiram may not be the same man as the Hiram you researched, it may have been a nickname or a middle name, and not used in the census.
If the age fits, and the unit was organized in or near where he was residing, and since the tombstone is union, I would think this would be your ancestor. Do you know James Capp's parents names or his wife's name?