Outside of exterior coatings,,if you store your car for long periods of time,,it's a toss up between safety storage and rust prevention,,,if you keep the tank almost full when storing,,then you'll never have to deal with rust in the tank,,,the problem is.....who wants a full tank of gas in their car when it's stored for a long time indoors???
Personaly i wouldn't powdercoat the tank,,,you'd be letting yourself in for a whole bunch of seam sealing problems,,,use a good rust proof undercoat instead,,,you can also have the inside of your tank coated with a sealant compound.