i've mounted my 9" solo seat several different ways now. looks like i've finally found the setup i like.
to answer the question, it was not difficult at all to make the actual mods, it really depends on what your plans are for the visual appearance of your machine.
a 9 inch chopper seat looks way to small with the stock fender. there is a 12 in at this site
http://www.crc2onlinecatalog.com/chopper.htmi'll be happy to send close up photos of the rear end.
currently mine is attached in the rear, with springs located along center line at front, and center point. it really fits the frame rails.
good luck enjoy
sluggo