Vintage SAAB expert Jack Lawrence of Motor Sports Services sold me the new nuts and washers, and also gave me some tips on how to install them. The goal of these tips is to let the nut slide smoothly across the washer, so the tab of the washer is less likely to break off.
Jack recommends first cleaning all the parts with a residue-free solvent, such as brake cleaning spray. Then, he says, put some dots of anti-seize compound around the flat side of the washer. The anti-seize compound lubricates the facing of the nut and washer, so the nut slides smoothly along it.