Rolf, this looks like a great project. But your last two pictures indicate your plywood stem mold may be installed at the wrong height (3/16" too low). Be sure to check this out with Northwoods Canoe.
Here's my thinking (a picture would be so much simpler, but I don't have one!). At the end of the flat run of the stem, it’s essential that the "top" faces (as viewed on the form) of the installed stem and adjacent rib be at the same height. The top face of the rib will be 1 & 1/16” higher than your plywood station mold (assuming typical mold stringers ¾” thick and standard ribs 5/16” thick). But it looks like the top face of your installed stem will be only 7/8” higher than the station mold (assuming your stem is typical 7/8” thick), because your plywood stem mold appears to be attached at the same height as the station mold. This is a 3/16" discrepancy. In other words, it looks like your installed stem face will be 3/16" lower than the adjoining rib face. They need to be the same height.
The best solution is to remove and re-attach the stem mold 3/16” higher than the station mold. If that’s not possible, you could fasten a 3/16” wood strip along the flat run of the stem mold, gradually tapering its forward end into the existing curve of the stem mold. In any case, the goal is to construct your form such that the top face of the installed stem and adjoining rib will be at the same height.
CAVEAT: my comments are based only on your pictures, and they assume typical w/c construction. Your situation may be different, or I may be missing something, so be sure to check it out with Northwoods Canoe. And good luck!