[turnpike boy]

It is best if you mount it vertically; if it's the can I'm thinking of, it has a typical radiator drain on one end, which goes in the down position, and a welded bung fitting on the other end. If you're going to use it as an overflow, make sure to vent the tank so that pressure does not build within. A small vent fitting can be used with a short stretch of hose so that fluids don't go all over the place should it become filled.

Overflows usually have a pickup tube (if connected to the radiator at their top) or fill from the bottom to ensure that what gets pushed out gets pulled back in as the engine cools. You can pull the drain and use this as the fill line - just be certain to vent the top of the can if you do.