There is a drop-in mask that comes with the camera that shows you the cropped out image area.
First, remove the viewer/finder from your Contax 645.
Second, remove the viewing screen (pull up gently on the tab on the right hand side of the frame).
The mask then drops in and sits in the area where the viewing screen rests.... Replace the screen on top of the plastic mask, replace the finder/viewer.
Now when you look through the viewfinder you'll see the edges are darkened down- almost as if you were cropping your image in Photoshop with the cropping opacity set to 90% and color to black. You can still see in the cropped area, but you have to think about it because it's so dark. Your eye and brain will naturally compensate and just get accustomed to looking inside the "cropped" area as your live image area.
You can still pop a film back and shoot without interference or cropping- but you'd have to remove the mask if you wanted to see the "full frame" without having to think about the fact that you're looking at only 70% of the live image area in the finder while shooting with a film back.
I hope this helps!!