First thing, ditch the air shocks. The shock mounting points were not designed to carry the weight of the car and can be damaged, if they haven't been already. Using them to enable pulling a trailer on that car has only made that more likely from the increased weight due to the trailer tongue, so be sure to thoroughly inspect both the upper and lower shock mounts for any signs of deformation or damage. You will encounter people who will swear, "I've used air shocks for years and have never had a problem." I'm happy for them having been so fortunate, but you will also find people who either have caused damage from doing it, or perhaps just have not yet discovered it.
With that in mind, remember what the real function of a shock absorber is. It is NOT supposed to carry the weight of the vehicle or affect ride height. It's job is to dampen then action of the springs, for example so that they don't continue to 'bounce' when they've been deflected. "Heavy duty" shocks are going to be more resistant to movement, so they're going to result in a harsher ride: you're going to "feel" the bumps more. Even though the original Pontiac shocks may not be available (I don't know, I haven't looked for them), you should be able to find quality aftermarket shocks that are at least as 'stiff' as they were if not stiffer. I'd contact some reputable quality vendors (KYB, Strange, QA1, etc.) and discuss your needs with their tech people. That's what they're there for.