You might bump down your audio -3 db so absolute peaks are at -9
If the audio is highly dynamic it might make it too quiet though.
No need to compress your AC3 to such a low bitrate, you should
set it for 448kbps.
if it’s stereo and not surround you can just use an exported .wav file
and have it uncompressed.
plenty of room on 25gb BD-R media for that if your film isn’t too long.