Whale, I had a plastic tank custom built in '04. Used the exact dimensions of the original tank as supplied by Florida Marine Tanks (who make or made aluminum tanks), which were 44" L x 31" W x 11" H (I left out the fractions). Sent them to Triple "M" Plastic Products in Maine, got a quote and had them build it. Here's a pic of it:
http://www.wellcraftv20.com/gallery/...d=P9240525_001
The tank, including a fuel gauge sending unit, was just under $500 not including shipping. I still have the drawings, if you want I can fax them to you, or try to e-mail them if I can figure out how to use my boss' scanner. ;)