It is about six years since I built my Night Heron, so I hope my recollection is correct.

There are several ways of dealing with the end stems, their attachment to the strongback and the forms at that point.

The forms are marked so that you can do a cutout and slide them over the end of the strongback. On the other hand you can butt them up against the end of the strongback.

The length of the strongback just has to be adjusted to maintain the 12" spacing to the third and third last form.

Both methods work. I have done it both ways at different times.