Where is the piece too long? I think that would make the difference in how you fixed it. If your piece is too long where you have your shaping then I'd think you would have to undo your bind off, frog down to where you started your shaping and work the piece again.
If your armhole matches up and it's just the part before you have your front shaping then you can take out your cast on row, take out a few rows and bind off where you need to in order for your sides to match up.
This is the reason that I always work the front pieces to a cardigan at the same time. I may have a lot of stitches on the needle (which is ok because I use circular needles for everything) but my pieces are always the same length and the shaping is always on the same row.