It depends on the slab. If you have a modernish house (built in the 80s or later?) you probably have a post-tensioned slab. You'd be fine bolting to this slab and cracking will not be a problem. If you are in a house built in the 50s or 60s (like me), you might be lucky enough to have some rebar in that slab. Our neighborhood was built with rebar in most areas of the slab but none in the garage, driveways, patios, front walks, etc. I have some pretty phenominal cracks in my floor. The most pronounced is in the garage where there is a good 1/2" gap. Maybe there's a civil engineer who would throw out an opinion.
Oh yeah, and you should use a hammer-drill with a carbide tipped bit. It's important to find a good one. I think the one i used one time was just painted to look like carbide or something. It took forever (but that was on tile, so you'll have more luck with concrete - it's brittle and chips a lot easier).
My safe also had holes to attach to studs and I did that instead.
ETA: I would buy a bigger safe next time around. Or maybe another two safes that size. I've got several other small closets that need a little something extra.
