I'd make them myself if I was building a car like you're thinking of. The biggest thing I think would be a set back would be making sure you get it all squared before final welding and during. I'd like to build a new frame for my 40, but I won't until I have a jig set up. And that 2"x4" 3/16" wall should be plenty strong. I'd think about gusseting or plating the joints at the Z's just for cheap insurance.