I believe that putting your longerons/ stringers in a horizontal plane will cause you to bend the stringers in more than one plane.

One of the reasons Yost's boats that I have built work well is that the gunwale is bent in a single plane (angled up towards the centerline.

There was another canoe builder who was suppose to put a permanate curve in his gunwales because the designed geometry did not do this. He didn't get enough curve, but when he put his frames in the correct position, it caused the gunwale to pull up due to a curve that was not balanced. His boat was hogged and he had to put spacers so at the frames centerline in order to keep the keel from being hogged. The gunwale was still hogged compared to the intended design.

If you let each stringer bend in one plane you will not have a vertical force resulting in distortion of the hull.

BTW, I personally do not cut notches in the frames to hold the stringers. I just mark the notch position, clamp the ends to bow and stern then pick out the middle frame and put a deck screw thru the stringer to the frame. This lets all the other frames fall into a natural non stressed position. One screw at each frame and you have established the shape of the kayak or canoe. If you don't like it it is easy to move the stringer. You really should fill any unused screw holes with epoxy. Also I epoxy each joint which is not typical with many people.

Good luck