You must be careful.
When we are talking about mixing parts on TTB lifts you just gotta do some checking.
Rules of thumb:
Most extended radius arms are made to be used with a certain amount of lift.
(IE Duff = 3-5", Rancho = 2", Skyjacker 4-8", Superlift, 4-8", etc)
The arm itself has the correct caster built in. If you dont have enough lift or too little it will not align.
Now I know guys who run Rancho arms with 6" of lift, and guys who run Duff arms upside down with 6" of lift, etc I have also seen Skyjacker arms run upside down and run 8-10" of lift. So there is and always has been alot of mix matching going on in the world of the TTB
Drop brackets; The same is true for the axle pivot brackets. they are built with a meaured amount of drop and in some cases the pivot point is also moved left to right.
This can be tricky and also get you in trouble.
If the bracket for a 4" lift kit only has 2" of actual drop, then the kit rely's on the new ball joint shim (camber adjuster) to correct the camber and make the kit work.
Adding a 6" coil to this kit would leave you with + camber.
+ camber on a ttb = dangerous and kills tires.
So be careful when you mix match parts.
My lift is made up of different manufacturer's and also uses custom beams to make it all work together, I have:
Superlift: 4" rubber brake lines, 2" drop pitman arm, Superrunner steering system
Skyjacker: 6" Ranger coils, extended radius arms
James Duff 2" axle pivot drop brackets
Rancho 9000 shocks
The only way to use a 6" coil with the 2" drop brackets was to build cut and turned TTB beams. it works well but I learned the hard way. If I could do it again I would skip all bolt on TTB kits and go with a custom beam kit. My next step is to ditch my stock coil buckets and shock mounts, I am going to run a 8" lift coil with only 5" of lift, thsi will increase my bump travel, which bolt on TTB kits seriously lack.