First I marked out the width at the front of the stem, and measured back from the front face to about 1/8" from the back. The curve on the front was much more uniform than the back, which was shaped to fit the inner stem - as best I could. The concave face is much harder to shape, especially when it needs to match another surface. In the end, I got all the gaps to within a millimeter or so.
Next I marked the profile for the transition on the front face. It is a section of a circle with a radius of approximately 2 1/2 inches. I used a template for the arc, which I traced onto each side. (By "template" I mean the bottom of a can of Raid hornet spray.) After that, it was high school geometry. Using a compass and straight edge, I divided the distance from the edge to the bottom of the arc on the front face into halves and then quarters. I noted where these intersected the arc. Then I used my Veritas saddle square to project their locations onto the side of the stem. Next, I divided the distance between the front and back faces in halves and quarters. I marked the intersections of these lines with the projections from the front face.
In principle, these marks should lie on an arc of an ellipse. They did not, so I marked a nice looking curve the passed near those points. After all, it's a boat, not a homework assignment.
Then I sawed down close to the marks to define the area to be chiseled out. I've already started a little bit in this photo. Then, worked my way down to the lines. I worked down most of the stem with my power plane, but it couldn't get in close to this little detail. So I used a hammer and chisel to remove the bulk of the waste. Then I used the chisel by hand to clean things up. Finishing up with a rasp, the smallest plane I have, and then sandpaper. I think it turned out pretty well.