The fenders should not be riveted on. There is a row of self-tapping screws wiht hex-heads along the inner edge of the fender (under the edge of the hood), one bolt at the bottom rear just behind where the mud flap is, one bolt attaching the mud flap strut, and two or three bolts holding the front lower edge of the fender to the "nose". These bolts are reached through the wheel-well and are just under the headlight bucket. You should be able to find where all the bolts are supposed to be by looking at the replacement fenders.

If you do find rivets, its possible that some PO did some creative body work to repair earlier damage or rust. One common area for rust to accumulate is the top edge of the inner fender, just under the side edges of the hood.