Check out this auction for an aluminum tank that would make an excellent tank for your biodiesel conversion.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=8011207370

You don't need a special tank since biodiesel goes directly into the vehicle's factory fuel tank.

This tank would also make a poor reactor if you were making your own biodiesel.

It might be useful to extend your range if you could make it fit.

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1984 300D
1985 300CD
1980 300SD
1993 SL500

I think ranchocortina meant a WVO/VO two tank conversion setup in the trunk of a sedan between the wheel house. It could also extend your driving range as a secondary tank as tirebiter says.