[dsraven]

there are also those who would just make the fenders wider to fit the tires. that would also mean a running board mod as well likely.
how wide is the box inside compared to the frame? it would be easiest to cut out and install some tubs. you can simply buy a steel trailer fender and cut off the width you need for the tub. it would then have a nice smooth radiused (?) edge on the inside of the box so easy on the eyes.fill the centre, where the wheel could normally be seen, and then weld to the box or flange the fender and bolt it to the box.