I would say some greenstuffing on the two parts of the torso would eventually be inevitable. I'd personally use the good old hobby knife for a slow, careful cut, and if I made a mistake, I'd cover it up with some other bit. That's how I usually roll, anyway.
Don't know about the heated knife idea - I'd suggest you first trying it on a piece of sprue to see just how well it works, to not risk damaging the plastic.
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