As far as tank types, there are 5 different types for EA81 cars. 2wd and 4WD for Brat's, wagons, sedans and 2wd coupes. The Turbo's had their own tank and the Hatch has it's own tank, either 2 or 4WD. For EA82 cars there is indeed only one tank. It has the dimple in the tank so that the diff will fit.
As far as fitting a 2wd tank into a 4WD EA81 car. It will require dropping the entire rear suspension to 4". Anything less will require massaging the tank if you will. A 2wd tank in the Hatch will give you about 1 1/2 more gallons of fuel. Not really that much considering the amount of work required.
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