Can you see where the drips are starting from with a strong light? You should be able to tell if
oil or
antifreeze by the color. Have you even started the car - hard to believe it would leak before you even ran the car and pressurized the coolant system?
Possibly it's the overflow tank itself. I would try to pinpoint the origin of the leak for sure. You may need to get one of those dye kits and a light down at a parts store if you can't make it out. I would not tighten bolts further. I think changing to studs and nuts might be a better approach as mentioned above.