I agree with Mike: use heavily weighted film clips when drying, then put the negs in sleeves under a heavy book (or pile of heavy books). I use a lot of Kodak Tech Pan, which is also a terror for curling, again because of the thin base. I have just been printing a load of 35mm nitro cellulose negs taken during WWII and kept ever since rolled up tight in film cans. They uncurled the same way, although that one took a while. If all else fails, once the negs are in the sleves roll them up against the curl and keep them that way for a day with rubber bands.