Are you seeing vignetting wide open or with the lens closed down a couple of stops? I agree with Rick here, most likely it's not the lens.
In general the "standard" focal length for 6x7 is 90mm, though there are some exceptions. For example, the 80mm Nikkor-EL and the Rodenstock 80mm Apo-Rodagon-N are both specified as covering 6x7. The rare 90mm Apo-Rodagon (non-N) is said to vignette (I have no experience with that one) but the even rarer 90mm Apo-Rodagon-N is specified for 6x7. If you're sure it's the lens then you could go longer, a 105mm will certainly cover.
If you're really trying to cover 6x8.5, that's more like 6x9 and you'll certainly need at least a 105mm lens for that.