+1 on creative steel for the differential bushing, as well as their diff block that goes above the diff while you're changing the bushing. I also have the creative steel motor mounts, which are stiffer and should last longer than stock ones, but add a small amount of vibration. Revshift makes motor mounts as well that come in two different hardness ratings, and UMI makes mounts that also have gotten good reviews.
You can test the clutch by driving about 50 mph in 5th gear, then push in the clutch, rev the motor a bit and then let the clutch out. If it grabs suddenly then it's still in good shape, if it doesn't then it's worn out.
06 Z06 - arh - wcch stage II heads - 520rwhp-489rwtq
05 CTS-V - mp112 - 90 mm tb - k&n cai - kooks lts 1 3/4 + cats - magnaflow - b&m shifter - fg2 shocks - ground control (600f/650r) - hotchkis sways - creative steel motor mounts, diff bushing & diff block - g-force axles - ls7 clutch&flywheel - nt-555 245/45 front 275/40 rear - hawk hps. 453rwhp-434rwtq.....12.4 @ 116 mph 60' 2.07 (SOLD)