The instructions actually say this...
If they're slightly short, don't worry about it? Okay...
I guess what I'm getting at is this, don't cut them TOO short, but just make sure you're around 173 1/2". You'll be trimming them anyway at 173 7/16". I cut 1/4" off each one to get the required 3/4" from the front of the skin.

I don't know how short is too short (I would guess less than 173" even), but if you're off a little, it may not be the end of the world for those longerons...
I just found that little paragraph in the instructions interesting.
