Rich,
That picture is exactly what I had envisioned. The ultimate bastard that will turn both heads and stomachs.
It sure sounds like the Nambu receiver will be a bit thin, but you might be able to get away with it if you re-tread it like you described. Just keep in mind that the steel quality in the late war versions is said to be a bit iffy. I don't know if that's true, but I wouldn't take any chances.