Target,
best idea is to remove the inner plastic panel and inspect. Keep in mind the fuel door and goose neck are there. You are going to use some sort of backer plate aren't you to distrubute the stress accross the fender panel? so you will need to be in there anyway.
One thing to keep in mind. Your antenna will have much better reception, if tuned properly, on the roof. Uses the whole roof as a ground plane and much easier to tun it up there. On the fender u will loose alot of distance with the same antenna as it will now be lower than the vehicle's roof.
If only using it on a trail talking to cars on the trail that is fine, but any long distance modulation u had on the roof will be drasticly cut. Also don't forget YOU WILL HAVE TO RE-TUNE THE ANTENNA. other wise you might cause some internal issues with the circuitry of your radio if the SWR is blown by moving it.
Is the roof top an issue with garage access and you don't use the radio all the time? I would use a quick connect and either remove it while city driving or a fold down and do just that. But the warning remains, YOU WILL HAVE TO RE-TUNE THE ANTENNA because you have changed the physical length with either of these products.
http://www.walcottcb.com/203ez-fold-over-adapter-p-262.html
http://www.walcottcb.com/firestik-k1a-internal-locking-quick-disconnect-p-571.html
good luck.