After I read your post, I got to researching this topic myself. Looks like the late 40's early 50's Chevy car tanks might be worth looking into.

Maybe mount behing the axle.

http://www.classiccarautoparts.com/images/pics/GM41-48.jpg
http://www.classiccarautoparts.com/gastanks.html

Chris


1954 3600 Chevy Truck
"The Fake Truck"
In the Stovebolt Gallery
More pix on Photobucket