The outpost is guarded by about nine hostiles- the only special one is the sniper on the roof of the main building. As usual, a good way to start a stealth run is to pick off outliers by sniping from the woods out back.
When you've taken out at least half, and the survivors reach the spread-out-and-look stage of the siege, use the main building to quickly ambush the remaining cultists. There are enough doors and rooms here that you can split them up easily, then take them unseen.
Take out the last to complete the outpost.
The safe is in the central building, on a platform up on the east side. Look for a hole cut in the ceiling of the 'office' with the answering machine.
Climb up (it may be easier when facing toward the table & chair). Up top is the safe.