Right! Welcome to Zdriver, BTW!
I remember that my original new '86 had gaskets so new that if the hatch release was pulled, the hatch would unlatch and sit there, held down by its own weight but not latched. It would bounce over bumps so I had to get out, go back and press it down to latch it. No clunking at all. My turbo had tired gaskets the day I bought it and I think I removed and reinstalled the rear gaskets reversed so there was more upward pressure on the hatch. I also looked at adjusting the latch position so that it pulled the hatch down tight and there was no 'play' distance for the hatch to bounce and 'clunk.'
Also check your rear mounts, visible up inside the rear wheel well. They wear out (your Z is getting up there in years. It's 30!) and can cause a 'clunk' in the rear at every bump. They are replaceable and new aftermarket mounts work fine. They are just hard rubber cylinders with a bolt cast through the center, fairly easy to install. That 'clunk' will drive you crazy because it never misses a bump, even the small ones. Cheers!
Your two rear struts are replaceable with new and aftermarket but they are not likely to help much in eliminating clunks because they tend to relax after use and will not exert enough, if any, 'pull' pressure to hold the hatch in place. Gaskets are your likely culprit and you may have to improvise replacements or 'renovation' there. 30 years is a long time to ask foamy rubber to stay resilient like new.
Struts are replaceable and you will want to do that when the old ones do not help to slow the slam of the hatch or hold it up when opened. That hatch is heavy and needs to be damped at close time
unless you like slamming stuff. The struts are the only help you get in holding the hatch up. Without them, you need a stick of just the right length. Replacement struts need to be checked to accommodate the weird ball/socket Nissan design for installation. Improvise, improvise!