I think that the 4.0 V6 swap is a great idea for a MII!
The 4.0L OHV 5-speed in my 1991 Ranger has lots of power and has over 160K miles. I had to install a limited slip rear just so my Ranger was drivable in wet weather. The later 4.0L OHC engines have more power, but also have more complexity.
If you need any measurements, let me know. I've already checked the Ranger vs. MII frame rails and they are the same at 24" apart. Of course the Ranger engine sits a little higher.
The rear sump Ranger 4.0L oil pan might work fine also, but I haven't measured to be sure yet. The shorter engine (vs a 2.3L or 302) could mean that either the lower pulley or the small front of the 4.0L oilpan ends up over the
suspension crossmember which would lead to a nice low engine placement. The MII rear 'toilet seat' brace might need to be notched for the rear sump though.
If the Ranger ECU is EEC IV, then the connections should be pretty much the same as the 5.0L EFI engine, so the popular 5.0L EEC IV wiring book could be useful. It should only require about 5 wires actually be connected to work.
If the OHC ECU is different, I would consider dropping back to the OHV engine. It has significant possibilities with a cam, headers, intake and head work.
I've seen pics of an early (German) Capri with the 4.0L OHC cam covers sticking through the hood, so its a tall engine. They should not be too tall for an MII, but I would measure before purchasing.
Hurst has a shifter for the Mazda 5-speed that all of the Rangers use that should cure some of the 'truck' shifter feel. The Hurst shifter has the same bolt-on shifter arrangement that the T-5 has, so you could probably use short T-5 handles with it. The factory shifter has an odd shifter attachment bolt that would not allow aftermarket handles.
The 5-speed in my 4.0L Ranger does not have the super low gearing (even with a 3.27 rear ratio) that you would expect from a truck trans. I think it would work fine in a MII.