What you have done is this:
Crop to 714x464
Scale to 880x480
Assuming the source is 16:9, you end up with this: (info from the AR calc found here - http://ps-auxw.de/cgi-bin/ar-calc.pl
Leaving the source at 714x464 (actually, I'd crop a bit more off the sides) would have been ideal, but if you have already edited then it could mean re-editing to get ideal results. The situation is salvageable, though.
You have a few options:
1. If you have not used many or any effects, including masking, which rely on specific things being at specific parts of the frame, you can re-render your source files using more sensible settings and have Premiere use those clips instead of the old ones (bait-and-switch method, find instructions here: http://www.animemusicvideos.org/forum/viewtopic.php?f=49&t=109175
). Doing so will likely take the most time, but it will ensure quality is maintained better.
2. You can also leave the video as-is at 880x480 and change the SAR. Before you start encoding with Zarx264gui, click on the advanced tab and change the "Sample Aspect Ratio" to x=22032 y=21596 (this is as precise as you can get using SAR in Zarx264gui). Doing this will take the least amount of work and you will keep about the same quality as you have at this point. The final output will not
be 16:9, but the image geometry will be accurate.
3. You can use Avisynth to scale the video to 864x464 resolution. This has the benefit of giving you accurate image geometry, smaller file size, and no need to mess with DAR/SAR/PAR settings. The drawback to this one is that you will be scaling yet again, which could reduce image quality somewhat.
Hopefully I have restrained myself from getting too pedantic here. All of these options should work for you, and two of them are very easy to do. If you would like further information, just ask. Good luck, and I hope this helps!