I wouldn't worry about several audio tracks hurting the video. Even at an average of 6500 I doubt the video stream will be much more than 5.5 GB.
448 kbps is massive overkill for a 2.0 mix. That's a 5.1 bitrate. 320 kbps is pretty much transparent, and even then that's probably too much. DD recommend 192, so I would probably encode 224, or maybe 256, to be safe.