Aftermarket fuel cell?
 

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The real question I have is why ?
Only theory I can come up with is anti slosh baffles for street racing.

Not certain about the 2000, but later years use dual fuel pumps to even out level and provide enough flow.
With all the monitoring and pressurizing that goes on with the fuel system I suspect you'd have to retrofit everything exactly to avoid MIL and other DTC's.

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Cool! Can't wait to see pictures.

I'd look through the Bentley Manual for all the fuel related R&R to get an idea of how it works.
There might be additional info in these SSPs that will help
http://www.volkspage.net/technik/ssp/ssp/SSP_377.pdf
http://www.volkspage.net/technik/ssp/ssp/SSP_217_d1.PDF
http://www.volkspage.net/technik/ssp/ssp/SSP_282_d1.pdf

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