I have an early 1974 260Z, smaller shock bumpers and the parking lights are under the bumper like the 240Z.  I am the original owner, I bought this car in San Diego in 1973 shortly after my honorable discharge from the Navy.  I loved this car, so glad I kept it.  It sat in my garage undriven for about 15 years.  In 2020 I noticed how these early Z's were becoming desirable and valuable. I'm near the end of restoring  it, just need the interior done.  Here is the big question to those who know the difference between the variations of the S30.  I am missing the interior rear quarter panels from my early 260Z.  I have asked the Zstore, Zcardepot and others what panels would fit? They were not sure.  My thinking is that it would be the 240Z interior rear panels.  If it matters, my Z has the bumper indentations on the fenders.   This is  my first post here...

I believe you'll find that the interior panels are going to be more like the 280Z (75-76) than the 240Z. If I can remember to take photos of my 260Z when I get home, I will post them. Then you can visually compare them to the 240Z trim panels.

Here's some pics from an early 260. Maybe these will help?

Another overlooked source for reference is the photo gallery on Bring a Trailer. 

https://bringatrailer.com/datsun/260z/?q=260z

It is home to countless 260Z interior photographs, many of which are unmolested original examples with super low mileage.

 

 

Of note are the vent holes above the strut towers. These look like speaker holes but are for the C pillar vents. 

The bump out for the expansion tank is larger than the early 70-71 cars that had the plastic tank. Also I believe the 77-78 were different as well. (Raised Floors)

Your 260 should be the same as late 240 (72-73) to early 280 (75-76) and the 260 of course. 1974

Bump outs for the taillight access too. Early cars were flat panels.