But why?
A custom tank has been on my short-list for years-- but the intent is to use the real-estate an IM has behind the beam (assuming one has such a thing, and I do) to get the fuel lower in the chassis and open up some more trunk space.
I understand that this is mostly just a project for me, as are most such things. A custom Spyder tank would pay for itself after about 1000 cars, assuming every car they went into was for somebody as passionate as Danny or Carlos or Ed. I can tell you with 100% certainty that not many people are.
I have found that when I'm building custom things, it's best to just assume you're going to be making one, and not 100 of the thing, and proceed accordingly.