A "smart thief"? Isn't that an oxymoron?
There was a case of a B&E in a nearby town where the would-be thieves broke into a vacation home during the winter. They worked for at least three days trying to get the Liberty safe open. Never got 'er done. I saw the safe at our local dealer, it was torn up pretty bad. They tried to saw it open with some kind of power saw, then cut it open with a torch. No luck. Company replaced the safe for free.
A friend had a big safe in his house. Came home one night and found his door open, inside the floors and walls were torn up where the thieves had evidently hooked a cable and winch, mayby a tow truck?, to his safe and dragged it out. Had it been bolted to the floor and walls they could not have tipped it to get the cable around it. Lesson learned.
I'd suggest you go to the local Ace Hardware store and ask them for hardened bolts that will do the job, bolts, nuts, washers and lock washers for the floor and lag screws for the wall. Expect them to be much more expensive than ordinary bolts, especially for the long ones to go through the floor. Then go to a steel fabrication place and get some steel plate for under the floor. I'd get two smaller plates, one each for front/back or left/right sides depending on how the floor joists run, rather than one big plate. Makes it easier to handle in the crawl space. Be sure measurments are exact to line upwith the bolt holes in your safe.