A 75mm lens is a bit short for 6x7, so you may be suffering from coverage problems. An 80mm lens is "standard" for 6x6, and 90mm is usual for 6x7 or 6x9, depending on the lens design. Thus, you may need to either get a longer lens, or adjust corner exposure to compensate. Check center versus corners with a light meter (without a negative in the holder) to check relative brightness. It should be the same across the whole area. You should also verify that you have the appropriate condensor for 6x7.