Does securing the tank upside down make any sense?
You don't need to, you got the one for filling tanks. But you can't weld
with this tank.

When you say don't overfill I assume you mean that there shouldn't be
too much
liquid in a tank?

Correct. If you put too much in you're going to pop something when
everything warms back up from the filling process. Buy a scale, eBay is your
friend here.

Karl