Clearly there are hazards associated with the residual fuel.
So here are a couple of thoughts. But please note I have not seen any MGF or TF tank or looked at drawings.
First, if you can see the rust is it not possible to clean the tank in situ one way or another?
Second if not and you wish to shrink the tank could you blow hot air in (down a hose from something like a hair dryer) and then “close” all openings and let it cool - but watch it cool to make sure it does not shrink too much.
it will probably  cool quickly.
Worth a thought?

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