What exactly is wrong with the fit of the chest piece?
For what it's worth, unless you don't plan on Bondo-ing the chest, I would wait to split it until after all the finishing work is done. I plan on splitting mine only until I'm pretty much ready for paint, to ensure that the ends of the cuts line up and fit together still.
Of course, it's kind of a risk since I never really got a chance to test fit the chest piece, so it better fit right because it will almost be finished. However, if you do decide to remake it, you'll know exactly how to scale it right based on the old one.
Good luck!