This is how your suspension should look factory in the rear.
Axle - block - spring pack - ubolt top plate: to help with towing you can add parts 3,4 & 10 which are overloads.
You can go with factory springs or for increased towing capacity have a local truck shop fab you up springs with an increased spring rate or order online from a retailer like Dever or Atlas. This will eliminate the need for airbags.
To start I would run a single block setup from PMF (Precision Metal Fabrication, on Google) and keep your factory springs and remove the factory block, lift block and add-a-leaf. To get a proper block height measure your factory block + the lift block + the add-a-leaf. Your factory block should be 2" the lift block 4" and the add-a-lead 1" for a 7" block that would have the truck sit at the same height. Decrease block height by how much lower you want the rear to sit. To run 35's on an Ex you should be at a 4" lift height to keep it simple. So the most I would lower the rear of the truck by is 3" otherwise a 4" block. If you measure you'll get a proper height. PMF does 1/2" increments. I'd also run PMF ubolts and ubolt plate to keep it simple. You will now need to change your rear sway bar links if you have a rear swaybar but the pic seems as you don't, if you do look for ones for a 4" lift either by PMF or else where. Your shocks also need to be changed for a 4" lift height. I would go with FOX 2.0's or Bilstien or BDS.
For the front.
I would buy 4" lift springs from BDS or Atlas with new ubolts. Since this is a 6" lift you most likely have a track bar relocation bracket and pitman arm. These will only work with a 6" lift. I would get a factory track bar bracket and a pitman arm from BDS for a 4" lift. Then you'll need an adjustable track bar to go with the factory track bar bracket. PMF makes a nice heavy duty one for stock to 4.5" lift. Front swaybar links will need to be changed for a 4" lift height as well as shocks and bumpstops. Brake lines might also need to be shortened. Ditch the budget special single steering stabilizer shock in the front for a PMF dual shock kit. The way it bolts is superior to all and will help keep the truck straight. You can pair it with their matching diff cover if you want some cheap bling.
I would also do maintenance items like balljoints and tie rod ends. A built steering box from Red Head steering boxes will also be beneficial.
Wheels and tires are also critical for a good ride. This is really up to you but i'd want quality! if it was my family while towing heavy. You can go as small as a 16x8 or up to a 20x10. For tires in a 35x12.5 i'd go with Nitto ridgegrapplers, exograpplers, Toyo RT, or Goodyears.
All of this should get you a killer ride and be in it for less then just the kelderman parts. Things like traction bars, overload springs, airbags, etc can be added later on to tweak towing capabilities.
If you don't want to piece together things, Carli makes a killer Excursion kit that sits at 4.5"s and comes with basically everything you'd need. Minus maintenance items and wheels/tires.
2002 Ford F350-Lariat-CrewCab-LongBed-7.3L PowerStroke Intercooled Turbo Diesel!
2.5 PMF lift on 325/60r20 Nitto M/T's
Enough Stuff To Pull Yours!
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