I have the 3.42 with the .82 TKO 600. Verrry nice
I have a fairly aggressive cam, so it does NOT like to run below 2000 rpm. I cruise at 65 mph at about 2200 in 5th, I think. The only reason to go with the .64 would be if you have a really mild cam, or if you intend to cruise mostly at 70 plus in 5th. I just wouldn't do it. There is really no upside unless you are determined to pinch your gas pennies.
Now you will hear from someone with a .64, but like Ernie says, it's all about the cam profile.
Sam