That's an easy fix. I take a heavy rubber mallet and whack the FSB as close to the barrel (so as not to bend the FSB ears) in the direction it needs to go. You can hold the receiver with your other hand and pin it down to your lap when doing this, so as not to bend or stress anything. If you had a carry handle upper, this would be super easy to hold onto.
Seriously, I have done it dozens of times on complete uppers I have bought straight from factories as well as my own I have built from parts. I usually do it so my rear sight windage ends up straight in the middle instead of off in one direction or the other like most end up when zeroed. It just spins the barrel slightly in the upper without moving the barrel nut. If a rubber mallet is used, it doesn't harm the finish either. I have seen leather and metal hammers suggested, but I wouldn't try that.
If you don't need it now, and you can wait, I would suggest the "Call DPMS" method above. I'm an impatient type of person once I have held the parts in my hand.