I wouldn't recommend it to you. Even if it's safe to do it now, the behavior can change anytime and there will be no one who's going to test it in future rathena versions.
So I'd do one of these:
1. Reduce the items in the storage. You can save all "deleted" items in another table and give them to your players though a NPC or something like that.
2. Test the current storage behavior. If it works advice your players to reduce their storage in the next ~3 weeks. Remove overtopping items from storages with more than 600 items after that time.
3. Wait for this Pull: https://github.com/rathena/rathena/pull/1115. If this is merged you should be able to keep your 700 slot storage.
4. Wait for this Pull: https://github.com/rathena/rathena/pull/1620. And move all overtopping to a premium storage.
it depend on the arrangement of your .@card variable that place at the arguments for getitem2.
it should be something like..
getitem2 ............. ,.@card[0],.@card[1],.@card[2],.@card[3];
and your enchantment item ID should be assign to respectively .@card variable correctly.