I would never rivet a panel on except just to hold the panel while welding,even then i use sheet metal screws. Rivets, cannot be grinded down smooth,which means a lot of bondo will need to be used and a funky looking body line. Rivets open up the oppertunity for moisture to get in and start rusting out your new panel. Im sure after a month or so you will be able to see all those riverts come through, just if you were to bondo over a hole, It will sink in and leave a divit. .
The only way I would reccomend putting a quater panel on would be completely butt welding it, then grinding smooth. In the end, you will need to use little filler and have a 1 solid piece of metal.
The only way to rivert the panel on also would be to over lap, i cant even think of how much of a hack job that would turn out to be. Butt weld... just my opinion