I had 2" blocks on mine for a few months. Never did go for a re-alignment but again, I only had them on for a few months.
For the 2" blocks, the bolt diameter was 1/2". The length from the top of the "U" to the thread ends wass 10.5". I didn't trim them although I could have because there was about 1.25" of threads exposed after the block installation. I wouldn't think you'd need much longer than that for the 3" but I would probably go a little longer cause you can always trim them after install.
Might want to think about trimming your bump stops while you're at it or get some shorter aftermarket stops. Although, I liked the way the Trac looked lowered and it only roughened up the ride slightly, I was tired of banging the bump stops even backing out of my driveway. Didn't want to trim mine or buy new shorts so I just removed the blocks and re-keyed the torsion bars.
Hope this helps.
Dave S.