DRY ICE! That would be both cheaper and more effective at shrinking the tank, than gas, you'd need to let it boil off before trying to remove the tank as it might make the tank too brittle, at the weight would make removal more difficult, but hopefully it would go back to it's original dimensions and stay there due to it's plastic memory.

The use of a plastic tank would've been motivated by the same old reason we have to suffer the fall out of stupid decisions.....Cost!  tooling for roto-molding is much cheaper to make and maintain than the stamping dies for pressing  out metal sheet, and all you have to do is pump the required amount of molten plastic from the extruder into the mold and multi axis spin it to use the centrifugal force to make to hollow form you require, where as you need to stamp out the sheet metal and them weld the two halves and any other shaped features such as the filler tube neck and any double skinning required like the gauge/ fuel pump mounting points, brackets etc. to a fuel tight seal!

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