I've had the spec 3+, which was horrible, chattered enough to break a poly mount. I had the LS7, but it had the rotational inertia of the moon.

So I pulled my tranny to have it rebuilt by sixspeedsinc, but due to shipping issues, I had to put it back together with the messed up trans. But I replaced the iron flywheel with an aluminum one. No chattering and it revs up like a sports car instead of like a Mac truck.

Also even with the spacer it engages very close to the floor. Definitely wouldn't recommend using the aluminum flywheel w/o the spacer.