I would ad the posi if it were mine.....but 3.42 is not much difference than 3.08....will depend a lot on your final trans gear and tire size...use a gear ratio calculator to determine your best options....
http://www.csgnetwork.com/multirpmcalc.html
In your other post your talking about a ls/od trans swap.....a 3.73 or so will be much better with the od