For the experience that is most like your current setup in terms of aspect ratio,angle of view, and general operation, I would say a 6x4.5 horizontally-oriented SLR with a 65mm lens and prism finder, loaded with 220 film for 30 shots per roll (similar to 35mm). This setup is almost exactly like shooting a large 35mm camera. You'd have to use Tri-X 320, though, as that is the only b/w film I know of that comes in 220 type. It will be about 2 degrees tighter AOV horizontally than your Nikon with a 40mm. If you use a 55mm instead, you'll be about 5 degrees wider than your nikon with the 40. Both lenses are options; you just have to decide if you want a little tighter than the 40, or a little wider.
These cameras go for dirt these days. I would get a Mamiya or Pentax, as they are super cheap. If you want a Mamiya, and you want interchangeable magazines, you must get one of the newer ones (Pro, Super, as opposed to 1000S, etc.). You pay a bit more, but this is the only big drawback of the older ones, and they took care of it with the newer models. If you don't need interchangeable magazines, I'd go for an M6451000S or plain M645. Try to find one that already has the lens you want, as you will pay a lot less in total that way.
I have no hands-on experience with the Pentaxes. I sure they are probably just as good. The big draw to the Mamiya system for me was the lenses, however. 80mm f/1.9, 200mm f 2.8, 300mm f/2.8, plus the handful of leaf-shuttered lenses, all of which work in even their new digital-capable bodies.
Here is why I say they go for dirt. For $380 shipped, I got the following:
-M6451000S with magnifier hood, with sportsfinder frame and mask, bargain condition, but perfectly working (Hood and sportsfinder are mint, though)
-80mm f/2.8 lens in mint condition with lens hood and Hoya HMC filter
-55mm f/2.8 lens in mint condition with lens hood and Hoya HMC filter
-Metered prism (non AE)
-Deluxe trigger grip
-Mamiya quick-focusing handle
-polarizing, 80 HMC, and 85 HMC filters
-120 inserts (2)
-220 inserts (2)
-Metal KME hard case, foam lined
-seller paid for installation of new foam ($45)
Shop first, be patient, don't let the money burn a hole in your pocket, and you will get a similar deal.
If you do end up getting one that happens to have a plain prism, and would rather have a metered prism, I will trade you for mine. I never use the meter anyhow, and would rather not have all the metering info on the side of the viewfinder.