Nothing is hard if you have the building skills and experience. If you wanted to tub it out (I don't)--you would either A--widen the rear fenders about 4 to 6" by splitting them up the middle and adding a peice This is not a big deal, I have done it before. or B---buying newer wider fenders, or C--unbolt the existing rear fender and fabricate a new wider spacer to replace the 1" square tube which I currently am using as a spacer between the bed side and the rear fender.
The running boards could be left stock, or flared at the rear to match the fender width. If you wanted to tub the wheelwells inward you would fabricate a steel hoop inner fender, cut the side out of the bed, cut the frame off just ahead of the wheelwells, weld in a crossmember, and fabricate a new narrower frame section for the area between the wheelwells.