Yeah, it will probably feel like your dragging *** everywhere you go. 12 bolts are considerably heavier then 10 bolts (and its unsprung weight most importantly). And its going to be much worse if the guy has a drag oriented diff in there like a locker, expect major understeer.
If I remember right a member on here a while back (LSGun i think) installed a 12 bolt on his car and hated it for handling.
Why do you want a 12 bolt anyway?

 

Oh I Didn't know it was a drag oriented diff . I though 12 bolts were just stronger than stock units. He is willing to part so maybe I can take just the Watts link setup. Will a torque arm be useful?

 

It depends. Since you want to handle you should keep a full length torque arm, otherwise the geometry is compromised under braking. You could consider a torque arm relocation crossmember to get it off the tailshaft though. What you choose will depend partly on what rear end you go with.

As far as the 12 bolt, it is heavier than the 10 bolt. But it is stronger. You aren't really going to get the light weight of the 10 bolt with more strength. I would say the biggest question mark is what differential is in it. If it is a locker or spool it will be useless for you. The other issue is with torque arm nuts coming loose on the 12 bolts, which is why some people bypass them for 9 in based rears.

 

Seller is upgrading to a 9 I believe that's why it's for sale. It's a eaton hd posi. That good?

 

Eaton hd is a great posi and isn't bad like a locker.

 

If all you are trying to do with the car is "notice a difference" then yes you should notice a difference