We really need a sticky on safes. Maybe even a section of the forum dedicated to it. Over on the sigforum we even have a safe seller who answers questions (constantly.)
Each of us approach the problem in different ways. The best way is to anchor it in a corner. A masonry walled corner. If it were me, I'd put a couple of 2x4 planks down and drill the holes in the safe first. Then I'd move it into position, mark the floor and walls, then move the safe back out and drill the holes. The lugs I've used in the past were Red Head brand. They're just studs that spread out as you tighten them down. I like the idea of having one bolt/stud in each wall (preferably the back and one side.)
For background, almost no crook breaks in through the door of the safe. That is by far the strongest part of the safe. Most crooks try to tip the safe over on its face or one side, then attack the weak underbelly. Remember safes aren't really safes, they're just thin sheet metal boxes. Most aren't even welded along their entire seams. Just short welds and then longer distances with no weld. Anyone with any metal fabricating experience can open one in short order. But first they need to approach it where they can do their evil deeds. If its bolted in place, they probably can only attack one side, with the others protected. Even the side they can access isn't convenient.
Also consider if you ever intend to move it, you might want to be less protective.
Consider if you really want fire proof. Nothing is fire proof, it only resists the fire for a short time. Most safes use some wall board (gypsum board, drywall) to do the insulation. When its heated, it releases trapped water, making steam, all over your fine firearms. Maybe a better approach if you have any plumbing skills would be to place a few sprinkler heads above your safe and gun area. Cool water spraying on the outside of a safe is a lot better than steam inside. Take your pick, neither is a good answer. Placing your own sprinkler heads are much cheaper than paying for the insulation and then shipping that extra weight. Insulation also takes up a lot of space you've just paid for.
Also, a safe isn't the entire security plan. You'd be well advised to install an alarm system. One that calls the police and everyone else nearby. Then screams bloody murder that its been violated. Outside sirens, strobes, whatever.
