Jools,
Dave @ Casoli had to make/modify a spacer for my ST4 when he fitted an Evoluzione slave - as I was having the same problem as you are now.
As to pre 2000 models - I've found that there's quite an overlap as to when they actually imported bikes with the later crank cases (as I found out when changing my nuetral light switch), so yours may still be the earlier type engine as mine is.
So..........a different size spacer may well be the answer!
Good luck with it mate,
Lee