Looking at the data, I conclude there are 3 major areas of vulnerability for a gun safe:
1. Pry bar - hammer attack.
2. Welding torch or plasma cutter attack.
3 Hydraulic lift - jaws of life tool type attack.
Of the 3, the 1st is the most common as it is the method employed by almost all burglar-thieves you are apt to encounter. To be successful, the the safe must be on its back or side with enough space between it and the walls for the crook to get ample leverage. The bolt down and placement of the safe are crucial in thwarting this attack. Helps if the safe is flush in a corner and flat on the ground to prevent the pry bar from achieving purchase.
Bolts will not prevent success from attacks 2 or 3 so they need only to be strong enough to thwart 1. Most recommend at least 3/8" x 3 1/2" bolts.
If you're worried about moisture from concrete, use a good chemical and/ or physical moisture seal on the safe bottom. I wouldn't recommend suspending the bottom above the surface because of the concerns addressed above.
IMO unless your safe has at least 1/4" solid steel sides and/or an advanced heat dispersing polymer construction your safe is not going to stand up to a type 2 attack and unless your safe is like a bank vault and set in cement, you'll likely be screwed in case of the 3rd type attack.
The best strategy might be to have a number of smaller type 1 resistant safes in multiple locations rather than 1 big expensive safe intended to resist all 3 attack types.
