I should have mentioned it's going over flooring so into floor joists and, I'm hoping, a couple in the rear to the wall studs. I'm pretty well stuck with placement so the floor holes are unlikely to match up with floor joists.
It's a cheapy Liberty safe that's probably around 400 pounds. That's the reason I want to secure it as well as possible knowing I'm only buying time for some tweaker that really isn't professional.
It'll be sandwiched in between two other safes and we're blessed to live in a low crime, rural area so I'm comfortable with the risk I've assessed.