I have recently replaced rear end of my car with all new bushes and springs and shocks and it still sags when i put my 2 sons in the back 1 is 7 and the other is 12 months old. I have monroe gt gas in the rear (second set of new shocks ) and still now different to standard.
I have lowered it with king springs all round and it drives better but as soon as i add any weight to the rear of the car it sags quite a bit.
other than modifing my air shocks to fit any others suggestions

How low is it? Superlows? Ultra lows? the Monroe shocks will be terrible. I would suggest getting a shortened shock like a Koni or Pedders shock. If funds don't stretch that far, maybe Boge or Ultima.

the spings are superlows. I have old gt monroes in my 66 xr and they are still like new, mind you they are also 11years old as well, these new monroes must be s**t then.
Had boge in a commode and they were crap, cost me $130 per shock and they were about as good a 1970s motor bike shocks ruough as hell.
So pedders are good i am guessing, never used them so not sure

I got pedders big bore shocks in the back of my car, they car handles awesome and doesn't sag at all with anyone in the back

Wrong forum lads.

moved to Gearbox, Suspension, Brake & Driveline Workshop

While adding HD shocks will improve control of the spring movement, ie reduce bounce ect, it will have little to no effect on how far the rear end sags with weight in the back.

Adding air shocks, or air bags, which obviously, can be pressurised to lift the car back to the desired ride height, is not something that can easily be done to react constantly to varying load weights. You would have to put the weight in the car, then pump up the shocks/bags. Once you take the weight out again, the back end will now be too high, and you would need to let the air out again.

It is probably possible to retrofit the Self Leveling Suspension from a NA-NC Fairlane/LTD, but these are usually removed for a good reason, and replaced with a spring, shock combo, like you already have.

The Monroe shocks really could be the issue. I had a set of Monroe GTs in EB sedan. They were the correct ones for a lowered car, but they kept bottoming out. I installed proper short stroke heavy duty Ultima shocks, and the wheel travel improved immensley. While the car still sat down at the rear with a load in, the diff was able to compress further and not bottom out on the shock.

Hope this helps in your decision making process. I am currently thinking through the same process for my Wagon.

Cheers
BenJ