Yes, the ribcase is the later 4-speed used after the Bugeye -- so I think 61-74 or so. Bolts up with no modifications (you may need a different engine back plate, but I honestly can't remember). It's an evolution of the smooth case that came in our Bugeyes originally. I used one in my last Bugeye, and my current Bugeye came with one already installed. It would be by far the cheapest option for you, and they are pretty readily available. The only visual difference is a few ribs on the outside case, otherwise it really is a "stock" option. Same gear ratios as the close-ratio option that was available in period for the Bugeye.
As for the Datsun 5-speed, it's a nice transmission. I've got one in my Morris Minor, and have one set aside for my Bugeye. Bearings and seals are still readily available, and some other parts as well. Other internal parts are getting harder to find, but it's a very stout transmission. The downside is cost -- the boxes themselves are getting more expensive and you need a conversion kit (I used one from Gerard's Garage, excellent quality) that adds another layer of cost. Still, that overdrive is nice and installing it in the car doesn't require any major modifications to the body shell so it's easily reversible.