You should also think about where to put the 6.9's oil reservoir in a 380 body.
I have seen a 6.9 in a 107 body, but I don't know how all the problems were overcome.
I wonder if a 380 trans can handle the torque a 6.9 will put out.

Yeah, I recall reading some posts from a fellow who did just that a while back, but I don't recall where I saw it.