Dear Les and mid-60s Buick long-roof fans,
Thanks, Edouard . I have struggled a bit with the unusual rear shocks (stud/eye mount on bottom and shaft mounted on top) discovering the there is only one currently produced shock for my '65 Sport Wagon which is produced by KYB called an Excel. It appears to be the old GR-2 non-adjustable shock selling for a very reasonable $20 from Summit. I found these shocks to be overly stiff on compression and lacking sufficient rebound to control the rear end "pogo" probably due to the extra weight and overhang of the wagon body and boxed frame.
Hmm, my wagon also has KYB shocks, but they were bought back in 2013. I don't remember which model it was, but I might be able to dig it up in all the paperwork of her restoration-modification. I haven't noticed any problem with the rear end bouncing as you say, but I never drive my wagon too aggressively. I'm not overly pleased with the car's handling at this time, but that's a problem that will have to wait until the drivetrain is back together and operating normally.
Let me know if you do find an alternative shock to the KYB. I kept the previous load-leveling shocks that she was equipped with before we switched her to a big-block, but they are old. If I ever get around to doing some serious towing I'll need some sort of air-adjustable shocks or lifting bags.
Cheers, Edouard