My own personal experiences must be different than some of y'alls, which I respect.
When I bought my '70 Monaco Bro, it had Monroe Load Leveler rear shocks on it. The rear end still sat low. The first owner had done some trailer towing with the car. I didn't know of any spring shops at that time, so my best option was air shocks. This was in '75 timeframe. I ended up with the Gabriel HiJackers. I got them installed and the air lines run and teed near the rear license plate. One line for both shocks. It took about 60psi to get the rear end up enough to level the car, no higher. Just to compensate for the rear leaf springs being "used".
The HiJackers have their regular shock section with the larger HD shock piston diameter and nice valving. What I was not expecting was the greater stability the rear end had in corners. Like a rear sway bar. As the car leaned to the outside, that side being compressed and with a common air supply to the other side, put more pressure on the inside rear wheel for better traction. I was very pleased.
Durability was very good. When the air bag started to seep, about 80K miles later, I got another set to replace them.
My '80 Newport had the factory HD suspension (the "no rear sway bar" HD suspension). It had done some trailer use, so it had some "Midas" air shocks on it. The bags were seeping, so I got some Monroe air shocks to replace them. Just need about 35psi as the rear springs are not sagged.
On the Load Levelers, you'll notice the assist spring is wound tighter in the middle than on the ends. That keeps the ride soft, initially, getting stiffer with more load. Compensating for "used" rear springs they will NOT, by observation. You'll not know they are there. As to increasing the load capacity? Not very much, from what I've seen.
Both the air shocks and Load Levelers are a "bolt-on" fix for the need for rear springs with an extra leaf or so, BUT my orientation is that the air shocks are the best option. There's more flexibility for the loading and still keeping the car level (up to their capacity).
The issue can be if a bag breaks with a loaded vehicle. The HiJackers have shields to prevent debris from getting to the bag whereas the Monores do not have shielded bags. Be that as it may.
In Ford's HD Trailer Package for their Country Squire/Sedan wagons, about 1970, they included rear air shocks in that package, BUT plumbed them separately. IF one went away, as I heard about from a local person, that side of the car "went down". If they had been plumbed together, the whole rear would have gone down. Neither is desirable, but one might be worse than the other one. So, paying attention to the rear air pressure IS important to help prevent sudden issues.
As an "aid", the air shocks seem to be the best option to me. With the Load Levelers, you get what you get. With the air shocks, you've got some flexibility of the amount of load you can add and still keep the car level. The need for minimum air pressure (usually about 30psi or so) is to keep the bags inflated and in place, I suspect, as that low pressure has no real affect on the car's levelness.
I will concur that using air shocks to raise the rear end of the car does affect the alignment geometry. It might be fixed on the car, but when you raise the rear of the car above specs, then the result is negative caster in the front end alignment. There can be possible stability issues with that much negative caster, although the steering effort would decrease a bit. Just NOT a good idea, no matter what!
Due to the size of the tube the front shocks go up into, the max shock piston diameter is usually 1", rather than the 1.375" for HD shocks. No clearance for anything bigger, so they make up the difference with the shock valving so everything's still balanced, f/r.
Back when the cars were newer, or in their "used car" era, there were many options for replacement shocks. Brands, piston diameters, etc. Now, I think almost everything is pretty much the same, other than whether the shock as low pressure, medium pressure, or high pressure gas in them. Monroe's MonroMatic used to be their basic shock, but now they have the larger pistons (when possible) whereas they all used to be 1" shocks. KYBs have a lot of good recommendations on them. Totally not sure about Gabriels any more . . . other than the air shocks.
I have no issues with using air shocks, even to compensate for sagged rear springs, but they were meant for temporary heavy load suspension aids. If you're going to haul heavy loads as a matter of course, get the rear springs themselves upgraded.
Enjoy!
CBODY67