By the time you upset a solid rivet, on opposite sides of a fender, you will destroy the terminal boards and dent the fender. You have to buck one side and smash the other. Not to be used in phenolic or plastic or fiber board material. Even a pop rivet may smash too hard and bust the board.

You got your tubular rivets too, which they probably used because pop rivets weren't used back then on automobiles.

Use a solid rivet with a tinnerman on the backside. (no smashing)
There are various styles of Push nuts. The upper left one is for straight shafts (like a rivet shank). Push it on the rivet shank, on back side of fender, with a small socket. It will give you the look and retain the board.

For fender molding
Lowes and many places have rivets. Don't look at the quantity of this stuff look at the price. You can't buy four nails either.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn