I agree, it must be an "interesting" box, with a Yale mortise lock on it, pretty big and heavy! It's the last lock in the world I'd use for any type of box.
Anyway, the three lever Yale is not hard to open, it's easy, but you would need some practice.
There are plenty of tutorials on lever locks, and tools required on this site.
You would really need to have one in your hand to have a look see, and to practice on, then you should be good to go
It's not a quick fix, you got some work ahead!
May be a lot quicker and cheaper in the long run to get a locksmith, guy would be in and out, job done, while you would be still thinking about it!!!