A little further on the ammo cans. They should have a synthetic rubber gasket in the lid that fits on the can when the lid is closed and locked down. These boxes can survive submerged in water for a considerable length of time until the box corrodes through. While I can see some possible minor problems with storing in a safe with gun solvents, If that lid is closed ain't nutting going to get in there. As far as storing in a safe, you bet your bippy they are in a safe set in concrete, hidden and alarmed.
Nice setup! Do you buy your ammo cans new? Online somewhere? All the cans I've seen at my local surplus shops are used and beat up.
Update: I bought a small, cheap fire/water proof safe last night. Its not as beefy secure as my gun safe, but it will have to do until I can afford to spend the required amount for a real safe. At least the coins/currency are out of the gun safe environment and on their own. I will be getting some desiccant and mylar bags to use also.