Hi,
Was the washer actually bolted up when you got it? If so, was it under the bolt heads and therefore only acting perhaps as an oil flinger, or was it between the sprocket and the cam flange and acting as a spacer? My parts book doesn't show it, only the cam, sprocket and bolts. If it was a spacer and you are using a different cam anyway, then if that cam had no spacer then I can see no reason to use it. It would be difficult to check chain alignment to see if it had been a spacer with your original cam.
I haven't ever found one in a dozen or so 400/4 engines or either of the 2 350/4 units I have.
Strangely the part numbers for 350 and 400 are different and there are 2 listed numbers for 350 and the sprocket has what appear to be balance weights on one version but no washer or spacer on either.
Ian
Forgot to say, I have a pair of thinned down mosquito forceps (with a ratchet lock) which I use for those circlips but the most annoying bit for me is the one in the master cylinder, although there was a post some place for a little home made bent bar used to hold the piston down.