I did this same modification and it worked out great and really cleans up the firewall. Just cut out the existing mounting location with a plasma and moved it over under the fender. Make sure that you have fender clearance before mounting. Also, we just welded an extension on the wiper arm and had to tweak it a little to ensure it cleared everything thru the full swing. Unfortunately, I can't figure out how to attach pics here but you can check out several pics of my build (in progress) which shows what we did.
http://www.cardomain.com/ride/756272