This was intentionally distorted to make the model's rear end look larger than it actually is and the lighting is adjusted in such a way that the shadows accentuate all the unattractive features.  Notice how the orange turn signals clash with the cherry red pearl paint.

Tools required: Patience, small hands, 10mm combination wrench, 10mm socket wrench (limited value), a dozen popsicle sticks, a utility knife, petroleum-based cleaner, patience, and a little Led Zep.

I purchased my J-spec tail lights from Nissan Motorsports in Olympia, WA.  
The whole set was under $400, including shipping.

These modifications took me about 3 hours.  It was not a highly technical conversion, just physically difficult in a few areas and an unexplored area as I could find no instructions from anyone who had done it before.  There were no explicit installation instructions with the J-spec assemblies, so most of the information came from my service manual.  However, I'm not known for RTFM.

The first step is top gain access to the back of the tail light assemblies.  The center interior cosmetic panel CAN be pulled down and laid flat on the rear deck without removing it entirely (though I had to lay a weight on it to keep it out of the way).  The small panels used to access the tail light bulbs must also be removed.  To remove the larger interior panels that run from the center panel around the sides of the trunk area is VERY involved, so it's best to try the change without going that far if you can.

Once you have opened up the panels, you must then:

1. Unplug the tail light assemblies from the wiring harness.  There is one plug for each of the three tail light panels.  You do NOT need to remove the bulb assemblies from the tail light housings, as these are part of the housings and are replaced along with the assemblies.
2. Remove the nuts which hold the tail light assemblies to the body.  There are 4 nuts for each of the brake/turn signal assemblies, and 6 nuts for the center panel.  The center panel also has a "press-fit" stud in the very center which pops out as you pull the center panel away from the body.

Note 2: On the left tail light, there is an additional component which is mounted to a tab on the assembly.  I think this is the flasher, but it just clips onto the assembly and must taken off to completely remove the assembly.  However, it does not have to be disconnected from the wiring harness itself.  (This picture is slightly out of order but it's the only one I had of how the flasher mounts to the left tail light assembly.)

Though my hands are on the small side, I still ended up with lots of scrapes on the back of my hands from attempting to reach the remote nuts like the one shown here.  Perseverence helped, and the fact that they ship all the replacement nuts you need with the J-spec tail light assemblies, so if you lose a couple it's no big deal.

This is the view from outside the car once you have removed the center panel with the backup lights.  It is the easiest of the panels to remove. Notice the areas around the two openings where there is a seal between the center panel and the car body.  A new pair of seals come with the J-spec center panel.

Note that you do not have to do anything with the lock mechanism (circled).  Clever design, eh?

This photo shows how pliable this sealant is even after 8 years.  Good stuff!  It really sticks to things so be careful not to get any on your hands or clothes or you will spread it to other parts of your car & tools.  I recommend that you have some type of petroleum-based cleaner (e.g., paint thinner) to clean your hands or tools after you have removed these two assemblies.

I used popsicle sticks (multiple) to scrape off as much of the sealant from the car body as I could, but I did not clean everything off as the residual butyl was still functional.  The J-spec kits come with new rolls of butyl seal so don't worry about trying to save any of this goop for later!

The bottom picture is the view after the tail light assembly has been removed.  You can clearly see the blue flasher unit that resides in the left corner of the body but is not removed during the conversion.
 

Now you are ready to starting re-installing the tail light assemblies starting with the brake/turn signal assemblies.

Yes I detailed the area behind the tail light assemblies.  Wouldn't you?

The butyl seal is a long cylinder that is rolled up with a piece of plastic-coated paper so that it can be unrolled as you apply it to the area around each brake/turn signal opening in the body.

Pity I didn't take a picture of the front of the center panel, but you'll see that later...

Notice that the J-spec bulbs are shipped with the assemblies. Since the J-spec bulbs don't work properly in the US wiring system, now is probably a good time to switch all the bulbs between the two sets of assemblies.   Remove the bulb sockets from the plastic tail light housings by turning counter clockwise about 1/4 turn and pulling.  You can then remove the bulbs from the bulb sockets by turning the bulbs 1/4 turn counter-clockwise.

Note that the brake light bulbs must be installed properly according to the little "nubs" on the sides of the bulbs.  The nub which is closest to the end of the contact tip of the bulb must be lined up with the deeper of the two slots in the bulb socket.  Push in slightly then twist clockwise to lock into place.  Now the bulb sockets can be installed in the housings by inserting the bulb through the respective opening and turning 1/4 turn clockwise.

Note 1: While you are applying the rope of butyl seal around the opening, do not stretch it.  It needs to be full dimension equally from start to finish to assure a good seal, especially at the corners.

Note 2:  The Nissan service manual (page BF-35 in my 1993 version) says to warm the area to 60C (140F) before installing the lens assembly.  I did not do this, but once I put the J-spec lens assembly in place, I used the nuts to force the lens assembly to the body of the car and set the butyl seal.  This may not work in colder regions and I would recommend using a hair dryer to heat things up slightly to ensure a good seal.

I also discovered it's best to do all three panels (left/center/right) because there are typically microscratches in the clear plastic of the lens assemblies that accumulate over time, and the black areas are no longer black but have a slight haze over them.  Since I did not replace the long horizontal piece that is in the trunk lid, you can see a slight difference in the "blackness" between that piece and the new J-spec lens assemblies.  I think it looks better to have the three lens assemblies all the same "blackness".  Hmmm...  Maybe I'll check into the J-spec trunk piece that also has the 'Fairlady' sticker installed...  Does it ever end???