I have tack welded sheet metal in those Mosler safes before. I just have the sheet metal pre-formed to allow me to use sheet metal screws. I find having the sheet metal raised 1/2" from the sides of the safe walls works well. I have used 24 to 16 gauge galvanized metal for this. Once in place you can easily secure what ever you want for the finished look including shelves.
You can drill and tap the sides of the safe to mount shelf rail brackets. It is slow work but it can be done. The sides normally do not have hard plate in them. The door near the lock and handle are another story.
I use a mig welder to attach the sheet metal. Make sure you have plenty of ventilation when doing this. All other normal safety steps for welding should also be used.
One thing is for certain. Nobody is easily going to pick up that safe and walk off with it.
You might consider just using sheet metal screws to attach the plywood or what ever to the original shelf tracks. This would give you something you can use to modify any way you normally would any piece of furniture.
Those are my thoughts on this project. Good luck.