I assume this is an API tank (either API 650 or API 620). In both cases, these tanks are not mechanically designed for liquid in the roof section of the tank (above the tank cylinder). In the rare case that a tank contains a liquid such as cool water, which is safe to overflow, then an overflow line can be connected at the top of the cylinder (near the roof seam). Overflow lines were relatively common many decades ago, but not today. That's because, for almost all tank contents, the risks associated with overflow are not tolerable.
If an overflow line is in fact safe in this case, then connect it to the tank cylinder near the top of the tank, and size the overflow line to be self-venting. If that critically important sizing detail is overlooked, an overflow situation can easily result in the tank failing due to vacuum.