Its not a Nissan engine, but if you go for a Cummins 4BT you will be rewarded with stump pulling torque and unsurpassed reliability. Its a 3.9L OHV 2V/cylinder turbocharged direct injected 4 banger with 360 ft/lbs of torque before 2,000 RPM in stock form. If you tune the Bosch VE pump it will probably put down more like 450 ft lbs with no mods. All that is required for the mod is a screwdriver. I am not familiar with the 4BT directly, but I used to own its bigger brother, the Cummins 6BT which is essentially the same engine with 2 extra cylinders. Everything else is interchangeable between the two and the 6BT engine was a beast. They even share the same 4.02" x 4.72" bore and stroke. You will probably need to do more fabrication to get the motor and transmission to fit and match up with the driveline, but you won't have any electronics to work with. All you need to worry about is the wiring for the starter, the alternator, the external voltage regulator (Unless you swap the alternator for a later model Dodge unit with internal regulator) and a simple SPST or toggle switch for the intake heater. Other than that everything is mechanical, so you don't have any messy wiring harnesses to worry about. The 4BT is a popular swap for half-ton American trucks like Bronco's, but its also been used in Rangers, Wranglers and the like with great results. They were originally designed for large delivery vans, construction equipment, and farming equipment. Cummins recommends 400,000 miles between overhauls for best performance, even with its non-sleeved block. With a claim like that, you know they aren't messing around.
Match the 4BT to a Getrag 5 speed transmission, pick an aftermarket clutch from just about any Dodge Ram diesel, bolt up a transfer case, fab some new driveshafts, fab the mounts, plumb all your lines, take the truck to an exhaust shop and call it a day. The downside is the 4BT engine alone weighs nearly 750 pounds, but its built like a brick crap-house. You might find that its too much torque for the driveline, but I've never heard anyone complain about having too much torque. The last downside I can think of is the redline. The engine will only rev to 2,800 RPM, but with a simple governor spring modification you can run 3,200 RPM with a brick on the gas pedal for weeks on end and the motor won't stop 'till it's out of gas. I think this engine is worth considering. They're reliable, powerful, and they sound great. I used to be a member of a Cummins diesel forum years ago when I owned that truck and there were people with upwards of 600,000 miles on 4BT and 6BT engines. The highest mileage I've ever seen that could be proven with documentation was 1.2 million miles. Mine had just over 300,000 and it started in half a crank, unless it was the middle of winter in which case it started in two full cranks.
When I had my Dodge 1-ton truck with the Cummins 6BT and a Getrag I could brakestand my 33" BFG A/T tires from a dead stop starting off in third gear. I didn't even need to touch the throttle to get them spinning in second, just hold the brakes and let out the clutch. It was even funnier when you're idling along in third gear and decide to stomp on the brakes. The truck would just stop and the rear tires would wail for mercy, even without hitting the gas. That thing didn't even flinch on a truck with a GVWR of 8,700 pounds, so I can only imagine what a 4BT will do in a Pathy.