For a rough approximation. you can double the FL of the 35mm lens or halve the 6X7 FL lens.
There will be a number of replies that say because the difference in ratio(1.5:1 vs 1.17:1) of the different format you can't do this.
As an example a 200mm lens in 35 will be a very rough approximation to 400mm in 6X7.