if it is empty of gasoline, then fill it with water, let it sit for a few hours then dump it out. open the filler neck and the fuel sender units and let it sit outside for a day or so, and it will dry out, (you could also use compressed air in there to blow it dry. after that, it will be safe to store. make sure you put the fuel sending unit back in, and cover the fillerneck with something to prevent dirt and critters from getting in it and possible clogging of it when you go to use it.
Could you define "safe"? You want to store it but afraid it might EXPLODE? Or that it might rust on the inside?
Eastwoods makes Metal Wash. Wash out the tank, rinse with water, followed by 1 quart of Acetone. Let it air out for day. Or, connect a hose to an exhaust pipe, the other end to the tank (empty of course), let the engine exhaust displace the fumes from the tank.