After a lot of thought and consideration, I decided to go with plastic for the battery boxes. They are made from 1/4" thick polypropylene plastic, welded at the seams. They will not corrode and will contain any acid spill that may come their way. Plus they are nice and lite.
There's room for 11 group 31 batteries in them there boxes. I'll be using 12 volt Crown deep cycle batteries, for a total of 132 VDC and approximately 18.5 KW of juice. Of course I'm going to have to build a steel support structure to hold them in place, and devise a way to hold the batteries firmly inside the boxes so pot holes and the speed humps in my neighborhood don't send them flying. Details.
Next up, I have to finish cutting the fuel tank out. I've been putting it off or working on other things because it is teh suck! But it has to be done.