Just get some wood under the cabinet if there is a gap below it, you can probably screw it to the floor boards from inside the safe.
Now just make sure there isn't a massive gap where the edge of the safe meets the wall.
After that don't worry about the rest, just get them to come and look at it. Most of the time they just hang off it to make sure it doesn't come away from the brick work.
Sure you'll be fine mate.