The shortest focal length standard-design enlarging lens specifically intended for 6 x 7cm negatives is 80mm. The 75mm lenses are intended for 6 x 6cm negatives (and can be used for smaller negatives as well).
When a 75mm lens is brought close enough to a 6 x 7cm negative to focus an image that covers a 16” x 20” print, the light will fall off a great deal at the outer limits and will begin to vignette the corners of the print. The greater the projection size, the closer you must position the lens to the negative. Due to the fixed projection angle of the lens, the larger the projection, the worse it will be.