When does it start engaging? Percentage wise, if that makes sense.. If it's getting close to 100% (all the way up), it's on it's way out. But you don't HAVE to change it till it starts slipping. Cruise at 35 mph, jump to 6th, and give it WOT. If the rpm's jump, it's slipping, you should replace it. If not, then technically you can still drive it. I doubt it'll perform like a brand new clutch though..
I vote to drop the ls7 in there anyways. You obviously want to, and you'll love it every time you drive. I went from a 46k ls6 clutch to the ls7, and it's awesome. I've got some new muscles in my left leg now, too. Man muscles. Don't get me wrong though, it's daily drive-able for sure.