This is going to sound like a plug, but I had the same problem. I had a 1Gb MP3 file that I was trying to convert to an M4b, only my MP3 to M4b conversion app couldn't handle the large MP3. I initially tried using Garageband, but it kept crashing. It seems it couldn't accept the file (on a new-ish MacBook Pro w/ 16Gb ram & core i7... Figured that should be enough for the basics in Garageband).
I almost bought a $9 app in the store, but I decided to go with the "free" option first. I ended up installing "MP3 Cutter Joiner". I never use free apps. I tend to be suspicious of them (ads, what else are they capturing, etc.), but this seems "legit". And I like that it was in the Apple App Store as opposed to something like SourceForge (which I'll use, but only as a last resort). Just figured while I was on the forum, looking for the same solution, I'd give a shout out to the app that saved my afternoon.
https://itunes.apple.com/us/app/mp3-cutter-joiner/id989083059?mt=12