I have the fiberglass flares. Will the Warfares fit without needing to do any sheet metal cutting when I replace them?

You should be able to position the Warflares in the same or similar position to your fiberglass flares. Sometimes, it is worth a little trimming to get a better fit. Ultimately, it is up to the installer to determine. The size and shape are similar, but the Warflares are much more flexible.