I had a similar problem with a roll that was left undeveloped from 1998, and developed in 2004.
Even after normal processing, and then 1 subsequent extra wash to try and flatten out the negs they were still very curly. I ended up cutting them and sleeving them anyways. Unfortunetly it seems, even 6 months later under heavy weight in a binder they are still SLIGHTLY curvy (make the sleeve page bend up when the page is sitting on the desk), Im not sure if this will ever go away but the extra wash, and weight added definately helped to remove at least 60% of the bend.. I wouldnt put your hopes up on completely flattening negs that sat curled for longer than I have been alive!