If Stoddard / NLA doesn't have the correct German style star washers, an Ace Hardware store will have a selection of generic ones in various sizes, you'll probably find some close enough to work. Ace sells them by the piece, not in sealed packages.
The reason to pre-soak the leather is to have a proper seal while adjusting the pump output volumes. Eventually the carbs in use will probably sit long enough for the fuel to evaporate out of the carbs, and the leather will shrink a bit. Then it takes a little while for the leather to swell again when putting the car back on the road. You want the pumps to have been adjusted for the operational state, not the dry startup state.
Most guys I've asked attach the pump actuator/adjustment rod in the hole nearest the end of the pump arm. To increase pump output, shorten the rod; to decrease output, lengthen it. A starting rod length to try is 3", but each carb will need its own particular length, it's purely trial and error to find out. One approach is to shorten the rod down as far as it can go, then start lengthening it, a full turn of the turnbuckle adjuster at a time, reducing the output until you are down to the 0.3cc for 2 strokes.
If you are not getting very close to the identical output volume from both nozzles inside a carb, it means one of the nozzles or its associated metering jet is somewhat blocked by crud. The tiny squirt hole on the underside of the nozzle is only about 0.011" in diameter.
Here's a simple tool I came up with for capturing the gas squirted out of the pump nozzles. It's a segment of plastic straw from 7-11 store, and a segment of a common pencil eraser for the bottom stopper, sealed into place with CyA (super) glue. The ID of the straw is 1/4", and the black mark is 1/2" above the stopper, marking the 0.3cc level. The notch in the straw (cut with a sharp Xacto blade) fits over the pump nozzle. You lower the tool down alongside the nozzle, then shift the notch sideways to enclose the nozzle. The gas will squirt down towards the stopper. Withdraw it after pumping two squirts into it for examination. Don't be in a hurry - when properly adjusted the gas from a full stroke of the throttle lever takes a second or two to stop flowing. Open the throttle fully, hold it open for say three seconds, let the throttle close, and repeat. Then see how much the tool captured.
When done using it, I slip a length of 1/4" wooden dowel into it to keep the straw cylindrical when not in use.