Good Evening, Cantore,
I also have a 75mm El Nikkor and use a Beseler MCR-X enlarger. That lens will NOT cover a 6 x 7 negative adequately, although it will come close if everything is well centered. You do need to have the condensers set properly to be sure light covers the entire negative; I've added a mark about 1/2 inch above the 2 1/4 x 3 1/4 mark on the scale for use in printing 6 x 7 negatives. Setting the condensers at the 2 1/4 x 3 1/4 mark would cause no problem, just very slightly less-concentrated light--a matter of no practical consequence.