The ducting is there to help warm your tanks in freezing temperatures. I guess you need to decide how often you intend to camp in hard-freeze conditions and run the furnace to keep them warm. Even if you decide this is a probability, it may make sense to change out the ducting for something with a better fit, or take some other option such as an electric warming blanket that sits under the tanks.
I recently went through the grey tank sourcing exercise. Spent several months mulling it over, and finally decided to go with the VTS tanks. Custom tanks are cost prohibitive, and there are few options if you want to keep the tanks within the frame and belly pan. Another forum member, Aerowood, used three 10 gallon (I think) tanks in his 69, and that may be an option that allows you the space you want for the ducting.