KYB Gas-a-just are a bit harsh for the light weight of a 6 cylinder coupe with stock springs, expecially if you install lighter wheels and tires. The KYB GR-2 is a good option here.
Koni's are very nice, just a tad expensive. Monroe gas shocks are pretty much like the GR-2.

If you are going to change to a V8 and stiffer rear springs then the Gas-a-just are a viable choice. I've got a set of almost new Gas-a-justs with brand new bushing set, for the rear (~200 miles), I'd be willing to sell the set for $50.00 plus cost of postage. I tried them on my convertible, and found they were just to harsh after removing so much extra unsprung weight.

Good luck, what ever you decide.

 

Although a viable sounding theory, the gas adjusts are just that......they automatically adjust, so not sure where you get the being too harsh theory. You can buy the GR2's in different stiffnesses, so don't just buy a GR2 without knowing how stiff you want them. If it were my car, I would contact KYB and ask them which shock they recommend for that 6 banger car. http://www.kyb.com/support/

 

It's not theory, it's practical experience running the two different shocks back to back on the same car, over the same course.
Gas-a-just shocks have a very stiff damping under quick shock shaft movement (i.e. high speed damping). That is why they are noted by many who have chassis experience as being harsh. This is also why I took them off my car, and put on the GR-2s. The rear end was coming loose under hard cornering on rough surfaces. Changing the rear shocks took the car from being a handful, to a real tractible and confidence inspiring ride. The GR-2s made a world of difference. The car sticks like superglue even on rough surfaces.
Interesting that when I mentioned this while talking with a bunch of guys who race Mustangs, they were unanimous in there agreement.

Where the Gas-a-just seem to work well, is heavy rear ends with a tall tire sidewall. The tire absorbs the initial harshness, and the weight of the rear end causes a slower mass acceleration curve, overcoming the shocks initial harshness, and playing to it's strength. A stock V8 Mustang with 14" and a 60 series or taller radial tire, and an 8" or 9" differential would be an example of a good use for the Gas-a-just.

The GR-2 is only available in one flavor for the Mustang. There is only one part number GR-2 (343219) for the rear of a '64-'70 Mustang, not different GR-2s for the same vehicle. Just like they only list one Gas-a-just rear shock for a '64-'70.

Scott