Hm. That doesn't seem right - a univarchar(600) would count as 1200 bytes. Perhaps it is a univarchar(300)?
There isn't a lot of wiggle room here.
You can create such an index if the column can be redefined to be a univarchar(299) or a varchar(598).
You could migrate to a server that had a 4K or larger page size; the max width of an index depends on the page size:
set switch on 3604
go
dbcc serverlimits
go
[...]
Item dependent on page size : 2048 4096 8192 16384
-----------------------------------------------------------------------------------------------------------
Server-wide, Database-specific limits and sizes
[...]
Max user-visible index row-size : 600 1250 2600 5300
[...]