I sell a larger 22.5 gallon gas tank that fits in 2wd Vanagons.  You do have
to cut the rear support for the tank and move it back several inches and
reweld it (tank comes with longer support brackets).  You use your old fuel
level sender as it measures the level not the volume.  These are $679.  I am
getting one for a list member right now if someone else wants to get in on
this order please let me know.
So if you were getting 250 miles on the stock 15 gallon tank then you would
increase your range to 375 miles (my numbers show you are only getting around
16.6 miles per gallon, yikes!)  A normally running van getting around 19 mpg
would have a range of 285 miles with the stock tank and 427.5 miles with the
new tank
(a significant increase).  You can see a drawing of the tank on my
accessories page on my website.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242