You need to set the Nero to allow overburning. Are you using Nero Burning Rom or Nero Express?
If you're using Burning Rom, go File - Options - Expert Features and let Nero allow Disc-at-Once Overburning up to 99:00:00. Of course, you'll have to burn Disc-at-Once.
Although the discs will be recognized as 703 MB ones, just burn as usual and ignore the warnings from Nero. Maybe you should verify after burning to be sure, non-standard discs are known for incompatibilities with some burners.