This thread is a little older.... I am wondering if the OP kept the F6 with the Contax 645 and their thoughts? If this were a current thread I would have suggested getting some Zeiss F mount prime glass for the F6 (e.g. 35mm f/1.4, etc) and enjoying the Zeiss quality and rich contrast that we all love in those lenses on the F6 and smaller 35mm format. In most cases if you just slightly underexpose your 35mm negative and develop by about 10/15% less in a fine grain developer like XTOL 1+1 along with the Zeiss glass you would find all sorts of reasons to keep your F6 and Contax 645. The same principles can be applied to your 645 as well. For large format I take the opposite approach by giving it more light.

Tim