normal red book audio doesn't allow for variable bitrate, or non-standard bitrate. When you burn, your authoring software will decompress your files to make them fit the spec (44100 hz, stereo, whatever it is), which is why they're called 80 minute CDs. There are 90 minute CDs, but they don't quite fit the spec. They're very hard to burn, especially on a mac, and when you put them in any particular drive, there's a very real possibility it could damage the drive.
I didn't answer before because I was hoping someone else might actually have a solution. you know, mp3 stereos aren't all that expensive anymore...