I originally had a 4.5" Skyjacker Class II, Double Flex suspension lift installed that easily cleared the set of 36x14.5"x16.5" Parnelli Jones Dirt Grip radials mounted on 16.5x 9.75 Weld Super Singles. Since taking up sled pulling I decide to make my own custom four link rear suspension to keep the axle wrap in check, I also fabricated longer front lower control arms and chose to get rid of the rear Skyjacker leaf springs and install some Firestone air bags. 
Suspension  Modifications
This is one of the two frame mounted brackets I fabricated to hold the ends of the links, its fabricated from 3/8" plate steel and bolts to the frame using grade 8 hardware.
This is a support/stiffener that I fabricated to bolt between the two frame brackets.
Here is a shot of the support/stiffener bolted in place between the frame brackets.
All of the link arms are constructed from 2" DOM with ½" wall thickness.
Front longer control arms in place
Upper front control arm
The brackets I fabricated to weld to the rear axle tubes
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Finished view of my custom 4 link, pan hard bar, and air bags
Here you can see the brackets I fabricated for the air bag installation
This is pretty much everything I fabricated for the rear 4 link with Firstone air bags and pan hard bar.
After grinding off the stock spring perches, and lots of aligning, I was ready to weld on the brackets I fabricated to hold the bars in place.
If you notice here, I had to redesign the air bag lower supports to sit off set from the axles as it was to close to the fuel tank.