um depends on how bad the rust is if its pitted bad i would replace them now if its rotted all over i would definately replace them.I dont know where you are located and if you have any swap meets where Luttys Chevy Warehouse comes too or not, i called Luttys up and had them bring a drivers side 1968 Chevelle Fender for mine brand new for like $319.00 no shipping charges thats about the cheapest you will find a brand new steel fender. Sometimes it would be better to go with fiberglass dont have to worry about dents or rust issues then use steel quarters, but if you put quarters on spray some corrosion resistant primer on them thats what im gonna do when i go to hang mine. Maybe even spray corrosion resistant primer on them with a couple coats of rhino lining on top of them. Also when you do quarters be sure to seal the back side of the lip between the quarter and fender wells. Thats about all i can tell you good luck keep us posted man.