If you can see the whole image in the viewfinder, I would suggest that it is the focussing screen that is out of alignment rather than the mirror but it would be worth checking that the mirror drops to a proper seated position after each shot.
Eric's suggestion of using frosted mylar in place of the film to try to focus onto is a good one. We use this stuff by the ton (or is that tonne?). If you would like a piece I can post some to you. I use fine textured polyester sheet in place of ground glass on my 5x4" camera and for various lens tests.