The best way to shave the washer would be to drill a hole in a piece of wood just big enough to put the washer into and only about half as deep as the washer is thick. Then just put in the washer and take a piece of sand paper to it and "viola" shaved spacer. BTW, not my idea. It belongs to someone else (I don't remember who) but it is a d@mn good one.

i did the washer thing for 2/3, for the same reason (save the white spacer), and i had stainless steel  washers sitting around, ran it for a while. eventually i went with the 1/2 white spacer. i used a flat file (rasp) and just ran the spacer up and down the file, checking occasionally to ensure a semetrically flat result. if you washer, i think #3 or #4, pay attention to ID to clear the needle and OD to fit in the recess in the slide, as they shouldn't bind.