They are probably Gabriel High Jackers. A good recommended pressure to keep them at is about 45 - 50 PSI. Any more than that and the ride is too stiff. I believe that the minimum is 35 PSI and the max is around 120 PSI but I could be wrong. I have been before. I have a pair on my 69. I keep them at 50. The ride is smooth and the car stays level. Click on the link below and you will see how the car sits.
How high are you trying to raise it? On mine, I just try to keep the body level so I only put in a little air. I have never checked to see what the pressure is though. Even at that it gives me a poor ride