Here are the images comparing the Dodge 8HP70 with a 4L80e.
Looking at the side view, you can see that the top profile of the 8HP70 basically follows the outline of the 4L80e.
The bottom view shows how the 8HP pan has a much smaller foot print.
From the top view you can see that the 8HP is a little wider in the area where the bellhousing transitions into the main art of the case.
The rear view shows the depth of the 8HP oil pan in the rear.
From looking at these pictures, I am pretty sure that the 8HP70 will fit where ever a 4L80e fits. If your particular vehicle needs to have the tunnel modified to use a T56 or a 4L80e, then similar modifications will be required for the 8HP70.
It also looks like the transmission mount will be very similar to the 4L80e location and to the T56 location. I will be using a TCP weld together crossmember kit. It has a universal transmission mount pad that needs to be welded in place.
Andrew