passenger side, rear of the block
You don't have a description of your truck & I don't want to scare you but ... ... ... Do you have a "53" block? Passenger side rear of the block is where the "53" block has a tendency to crack. I hope I'm wrong but I would look very closely for a possible crack running over to a tapped plug & making it look like it's the plug leaking.
Good luck, & again I hope I'm wrong.
Clay