Originally Posted by retinax
OP, people are understanding your question in two different ways. Are you looking for a lens to use on 135 or for one to use on 6x7? To me it sounds like you want to know the closest in 135 to a 90 on 6x7. And that's a 50. It's not hard to calculate yourself: the ratio of the long side of the frame (or short or diagonal if that's more important) to focal length is about the same: 70:90 and 36:50 mm.
I think I'm reading it as he has a 90mm on a 4x5 with a 6x7cm back, and wants to know the equivalent on 135. So in reality he's asking what the equivalent of a 90 on 6x7 is, really, no need to even think about the 4x5 coverage. So I'd agree with you.
Seems to make logical sense for me: an 80mm on my Bronica SQ is about as close to a 50mm on 135.