Not to take away from Martyn but needing that much width and knowing enough to guess that the axle you are dealing with is probably only rated 1000-1500lb and going to have very small wheel bearings and spindles. Putting that much extension on the axle is going to increase the risk of bending the spindle or burning up the bearings.
I would concider changing the axle out. It would not be much more then a set of custom spacers if it is a solid axle and you can order it the width you want. It would be much stronger then what you currently have. I am sure Martyn could source an axle for you just as easy as making the spacers.
You might be able to find something suitable local as well. You could probably re drill a trailer drum to the pattern you need.