If you are going to have air bags installed go to a reputable spring shop, see if they do airbags, and get your springs resprung while they have it.
If you are doing it yourself well that'll probably solve your sag...
Dont know if this has change but the f250 rear blocks use to be around 1-7/8" tall vs the f350 blocks at 3-7/8" tall. If you want more capacity f350 with overload springs, more height no extra capacity change get taller rear blocks. If you want to deal with the air bags that's a personal choice nothing against that, Simplest taller blocks
Air bags installed and next day had a salting event. Air bags 60psi. Went to the scales,front 4120,rear 3640 toal 7760. loaded a yard and a half salt. there was no sag and went back to scale. front 4100 rear 7100 total 11200.
2015 f250 supercab.10000 gvwr. I know i'm overweight on rear axles but tires are rated for 7500lbs