Been there, done that on my '98.
Providing those side arms aren't modified...
To fit the wingrack to the earliest 98-99 bikes, you need the pillion hangers from later 2000-03 bikes, the pegs have a rubber base and sit about 35mm lower.
Once you have the later hangers fitted, it should all fit with a fair bit of jiggling (and re-jiggling). Don't bolt any part up tight - you'll almost certainly have to undo it until everything is fully lined up.
Then you'll find that with side cases fitted, there's barely anywhere for a pillion to put their feet ...but we'll cross that bridge should you come to it