They make the 50gal verticals for behind the box. I think all of the 100 gal "L" tanks require 8-13" under the box. UWS is the goto brand in Florida, check around though, I've found the boxes built in your own state/area are a bit cheaper.
Like Reax states, their is manufactures out there that make tanks that are " L" shaped to put your box onto . Grainger , Northern Tool , Transflow are some that stock them . I had one built for me ( it is " L " shapped holds 110 gals fuel and has a 30"w x 30"h x 48"L tool box that sets on it from Taylor Wings .They build alot of big truck boxs, racks , chain holders, cab wings ( look at my gallery for mine ) . They are in the Sacramento Ca area .