There are a couple of ways to consider "normal" focal length. The typical method is to use the diagonal of the film, which for 6x10 would be about 135mm. An alternate approach for panoramic formats would be to use the width - 120mm for 6x12. As you lean toward wider views, that might reduce to 110mm or even 90mm.

If you don't like the idea of having to change lens cones with the Fotoman, you might conisder an inexpensive 4x5 Crown Graphic with a Graflok back that would allow you to use something like a Horseman 6x12 rollfilm back.