Thats odd, as when i fitted a 200 flange to my 300 rear diff, there was a spacer in the bag with the flange, but when i fitted the flange, i didn't need to fit the spacer...... however, mine is leaking, so maybe i did need it after-all...LOL.....
If you're changing the diff anyway, the easiest way to sort it is to fit a FRONT diff......