The old fenders would be the correct shape and thickness for repairing the "new". If the old fenders are dented in the spot you need to use for a donor, fix the dent then cut out for use on the "new" fenders. A light coat of body filler is totally acceptable. The Bondo brand is not considered the best to use. read up on it all on the web. Good luck, it's a great pleasure to fix up an old truck.

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