Well, if you used a Globeliner as a basis, you COULD still make it a 6x6 by using a TLT axle on the front. Then, you use the inside half of a dual (rear) wheel on that axle, and it makes it about the same width as the rear axles with the dual wheels, (about 7" wide).
Now, another option using these same axles, (and I'm doing this on a Pixar Mack Hauler I'm making for my grandson), is to use "wheel spacers" on the rear axles, which will bring all three axles to the same width, (about 8" wide). The type I used is similar to these:
http://www.ebay.com/itm/4-12mm-Hex-W...oAAOSwB09YPii5 -Mike