dave84 - I bought mine from Rock Auto for $24.79 each. So the Summit price looks good.
Phillip - KYB did not have a direct replacement in the Gas-assist for the 344052 so I went to their webpage and found an Excel type spread sheet with their shocks that had all the dimensions and end configurations. From that list I found the KG5429 to be the closest. I seem to remember it was a front shock for an 80’s Ford Econoline van. LOL So any auto parts website will not show that it fits a 68 Cougar. Again this was something I researched and after a year am still happy with the results whether it is considered correct or not.
It also shows RWD Checker Marathon Rear shock for 74-82, looks like its one of those elusive front AND rear. WTF is a Checker Marathon…
Old Yellow cabs.
lol oh.
Great info - thanks again 
- Phillip
I got this from NPD
The Mustang shock is 17.6875" fully extended. The original Cougar shock was 18.75" : 1.06" longer than the Mustang. The AMX shock is 20.67"
The Mustang Shock was 10.5" fully compressed. THe original Cougar shock was 11.1875" .69" longer than the Mustang. The AMX shock is 12.06"
The Mustang shock is shorter. If you lowered the rear of your Cougar it might be a good choice. THe AMX shock is longer and a good choice if you lifted the rear.
NPD has anew but expensive shock that is very close to the Cougar, 18.23 extended, and 11.23 compressed.
Bill, This is good to know for others in the future depending on just how expensive it is. As I stated earlier, I purchased the KG5429
to try as an alternative to the AMX shock. I was willing to spend $50.00 to find out and if they didn’t work I could put the old ones back on. I purposely did not update my post on this thread at that time because I felt that if they failed no one needed to know. Now after more than a year being installed and over 3000 miles, I am still very happy with them and that is why I felt I could let others know. Again nobody has to follow my path but I thought as an option for those looking for a firmer shock than the GR2 344052, there is a gas-assist by KYB that works.
P.S. - over 600 miles were on the Dearborn trip which were well worth it!
Okay this is the new Shock from NPD. It is expensive but it is also pretty cool. There are two knobs at the bottom for adjustment. One is for compression and the other for rebound. They are MADE IN THE USA machined and rebuild-able. Priced accordingly… $189.95 ea
That is a hell of a piece.
Installed the KG5429 rear shocks yesterday and they fit perfectly for me as I’m 1" taller out back. They extended a little bit past the shock mounting plate so I have full travel. Good looks on the information.
dave84, Have you driven your car with the new shocks yet? Just wondering how you like the firmer ride they provide.
It looks like a repurposed QA1 shock.
I haven’t yet Mike, still have some projects before I test fit tires which is the last stage. The installation was very smooth though especially for me with the new taller leaves.
I decided to go with the KG5429(higher performance shock) on the rear over the 344052(The well known AMX shock). I figured that the shocks will most likely be there for quite a long time and I don’t want them mushing up on me. There is also the better chance of maintaining ride height over time and other personal reasons. I attempted to go with KG4517(supposed to fit) shocks in the front to match the rear but they were too short. I had to go with KYB 343146(OEM) but not bad considering there is more noticeable bounce from the front of the car. I have not had a chance to hit the highway yet but I have to say my 68 vert rides much, much better now. I gotta get accustomed to the feeling of better control over my old cat. 
Throwing in my 2 cents here. I used a Bilstein shock (made by Race Car Dynamics) from Street or Track. These are sized for a Mustang. I also removed the big rubber and steel spring perch/clamp setup when I installed my Flex Form composite springs. By doing this the shorter shock works well. Damping and higher spring rates work well together and no bottoming when loaded down. I’m very happy with the changes and weight reduction with the new rear springs (10.5 lbs per spring).