Miles,
I have the Billings Samson II ABS kit, there are some photos on here somewhere, posted by Martin, which you may be able to find.
My hull has a small flat return at the top, no instructions about trimming the hull.
But after going through the instructions a couple of times, it became clear that a timber rail was fixed to this return, so the hull was OK as is, no trimming was required
Might be the same with your Smit, so go through all the drawings etc a couple of times.
I take it your kit has a full size plan which will also show the "finished" hull shape.