Save yourself a big headache before you rebuild your shackles.
1) place a piece of 1"X4" between the cab and the frame near where the shackles go. A 1"x4" is 3/4" thick. That will level the rear of the cab exactly like what brand new shackles would.
2) move to the front cab mounts and check for collapse cab mount supports. It they are not collapsed, shim the front cab mounts to specs.
This should give you a perfectly level can
3) put the rear shackles on and remove the 1"X4"s.
You shouldn't need to add much in the way of shims for the shackles if they are in good shape.
