You're overlooking the fact that 24mm is 24mm is 24mm. Any 24mm lens mounted on your camera will give the exact same FOV. A 24mm Mamiya lens will give you the same FOV as a 24mm Nikon lens. If the 24mm Nikon lens isn't wide enough, then switching to a Mamiya 24mm will solve nothing.
Not in this case! The Mamiya 645 24mm is a fisheye
lens, and thus will deliver a substantially different, much wider FOV than the rectilinear 24mm Nikon.
[Google led me here, so although it's an old thread I couldn't let the "facts" go uncorrected]