For the inside of the removed fuel tank (with the sender removed) I would firstly give it a swill out with half a gallon of clean petrol to remove any loose deposits (you can store and re-use that fuel for cleaning other stuff so it isn't wasted), then I would use half a gallon of acetone based cleaner inside the bunged (sealed as best you can) tank giving it a gentle swill on occasion then flush it with very hot soapy water. After this, flush it through with cold water from the hose until you are satisfied all detergent is removed. Lastly, be extremely patient by drying the inside with a hot air gun down the filler to dry it out. Try and store it somewhere where the inside wont condensate and rust until you need it.
Hope this helps.