As already mentioned you clearly have a pair of forks for the later model Monster or possibly the ST range but either way they have the wider caliper spacing as you have already found out.
However the fork spacer issue you have discovered is due to the later bikes having a larger diameter axle which you will need to source along with doing something to enable you to fit your front wheel as that will be for a 17mm axle.
I know changing the bearings to suit has been raised before but seem to remember it’s not quite that simple.
I lifted this from the ‘other’ forum but it’s got a pretty good pictorial of a later set up which might help you see what you’re missing/need.
http://ducatimonsterforum.org/index.php?topic=10276.0