Garageband - Remove Dead Space at End of Track
By saying cut off all of the dead space, I assume you wanna crop the track to remove silence at the end of the track? If I'm right, here is 1 solution:


Add the track to your iTunes
Right click on track and Get Info (or just cmd+i)
Select Options tab
Tick stop, and select time you want track to end and press ok
Go to iTunes Preferences: cmd+, (comma)
In General tab, click on Import Settings and select your output format, eg. MP3, then ok
Click on your song, go File > Convert > Create MP3/AAC version
When this is complete you can find this track by right clicking on it and say Show in Finder


Hope this is what you wanted? If not, it might help someone else as this option has been silently removed from the right-click context menu. :) Let us know.