Yea. I was looking at the 70 swap as well, but realized quickly it costs just as much, if not more than just building the 51, when you really start doing the line items.
Other than the weight, you just described a M340i/M440i xdrive.
They are now mild hybrid, both engine and transmission, doing exactly what you described.
M240i still has the non mild-hybrid engine/trans, like the Supra.