Bill, they can be installed with the fenders on. Don’t have a pic handy but I think it’ll be pretty obvious one you have the splash shields with clips in hand and look up in there behind the headlight bucket. There are little holes where the clips snap into.
I’m happy with the stuff I get from Restoration Specialties:
1970-1971 Plymouth 'A' Body Front Fender Rear Top Splash Seal With Clips
Download their pdf catalog.