I don't know about the shims.
You get half of the extra shackle length in lift due to the fact that you are only lifting at one end of the spring. If you were to lift at both ends of the spring then you would get the full benefit of the longer shackle.
This is a lift idea that I am planning to implement in the coming weeks. It has it's draw backs in that you are more likely to hit your shackles and spring mounts on obsticles, but it can be cheap and relatively easy.
I plan on using possibly 2" longer than stock shackles, and then bolt on 2" longer than stock spring mount extensions at the other end of the spring. That will give me a true 2" lift without any change in caster or pinion angles.
Should be interesting......I might even go with 3" shackles, and a 2" extension for a 2.5" lift, and later slap on some CJ springs. I'll post up when I get around to doin' it.
For this weekend's run, I'm going to remove the bumpstops.......I've noticed that they limit articulation badly.
'77 CJ5 Fozzy Locker
20 degree RTI 1250
'87 Samurai Stock (for now)