Glad you like your safe. If you aren't on a slab, I can tell you with absolute certainty that you're going to need to support that safe from below. Sistering a joist isn't going to be sufficient. You're actually going to have to built a weight distribution column to a what essentially will be a weight bearing floor joist. Failure to do this will have your floor joists bowing in no time. Especially if some moisture were to hit your basement.

Dang that thing is heavy.

Tom