Here are a few pics of progress so far. As mentioned earlier, the butt plate and toe plate are fitted to the stock, and the tang has been fitted to the breach plug on the barrel.
The crescent shape of the butt plate in no way matched that of the stock, which was left with the afore mentioned ridge in the middle of their mating surface, and the mating surface of the butt plate was far too rough in its "as cast" condition to fit anyway. The toe plate should have been easy but, alas, it was too short to reach the butt plate once the "inletting" for it was cleaned up enough to accept it. I wound up having to build up its aft end by welding. But, they now fit.
Speaking of "matching" (or not), the octagon of the tang in no way matched that of the barrel. The mating surfaces of both parts were anything but "flat". Both required extensive filing and grinding to make them mate up. Beyond that, the "hook" on the end of the barrel, on the breach plug, was too big to fit through the mortise in the tang. And its vertical sides were not even parallel, nor precisely "vertical".
None of this is insurmountable, of course. Maybe inexcusable, though, especially in a kit of this price. And this is the "easy" stuff to start with. It gets much harder from here.