Many here run air shocks without problems. Haven't heard any reports of shocks breaking off or punching thru the mounts...anyone? The mounts don't carry all the weight of the car, they just carry a portion of it. So it can work. If you have a solid unibody, I wouldn't fear trying it. But it is indeed a band-aid, and not the proper way to do it...same with ANY other simple solutions besides having correct springs on the car...air shocks, helper springs, longer shackles, shimming the front spring mount...All will raise the rear. And all are an incorrect compromise that MAY (or may not) cause other problems.

The BEST way is to replace with the proper springs. Or remove your current springs have have them re-arched or have a leaf or two changed out by a knowledgeable spring rebuilder.

Your problem is unique if indeed the springs were replaced as you state. Far more often, people here who replace springs on these cars achieve a rear stance that is too high...not too low as you report. And in today's world, everyone seems to want the ground-hugging stance anyway. Rather rare to find someone who wants to raise it like you do.

I agree pics are worth 1000 words, so others here can weigh in as to whether the car is "supposed to be" that way, as per your shop.

But even at that, realize that there was more than one spring option for this vehicle straight from the factory. For example sprints and HO cars got "heavy duty" springs as part of their package. I'm more familiar with 68 than 69. But for my year, many models could get "ride and handling package" which had different rated springs from normal-issue. So I'm fairly certain ride height varied on many models right off the showroom floor. Plus the aftermarket and repro market offers MANY different rear spring rates, allowing everyone to customize their ride heights and handling characteristics. So what is "right for the car?" Seems to me that "right" frankly it is, and always has been, whatever you want it to be, ever since day one when Pontiac allowed people to choose suspension options...