OK, test is done, good enough for basic pimping purposes.
Conclusion is quite simple, full view in the finder is larger than actual on negative, and the corner marks are for 645, except also quite outside of actual negative.
Corner marks fall about in the middle between 645 limit and full square negative. So using these marks as actual cropping aid for square 6x6 gives some margin of error, to me safer way in most cases. Where there is a need to take advantage of full negative area, the only way to go is with focusing screen adapter, finder is not good enough.
Pics below are not perfect, would have to toy around with proper D set up to get, especially finder view better, but show enough I think. In the finder you can see area outside of target (shown as on-screen), that's how much more you see in the finder vs. on the focusing screen. Note that pic for finder view taken after dropping camera to compensate for parallax.
Zoom into finder's image to see better.
Grid lines on the focusing screen match 645. In the pic they are not exactly aligned because of the pain getting the whole thing aligned.
Side note, had 3 bubble levels on this set up, tripod head itself, Hasselblad quick release plate and camera. They never came together, just in case anyone wants to argue their application for critical work, i.e. testing.