The new standard Kirkham differential ratio is now 3.54. Before that it was the 3.42. There is not a lot of difference between the two.
I like the Tremec TKO 600 (purchased from Joe from Standard Transmission -
http://www.standardtransmission.com ) They have been great.
I have the .82 OD, but if I had to do it again, I may change to the .64 OD. I am rarely on the highway and so I rarely use 5th gear as it is (engine likes 2000 RPM and higher due to big cam).
But when I am on the highway, it would be nice to drop the RPM's a little more than the .82 provides. But I think it's a risk, a .55 or so would be perfect in my opinion, but it is not an option.
That being said, I am not spending the money to convert to .64 OD (I was quoted ~$600) since I have doubts that it would be too low a ratio.
Hope that helps.
ps. I made a chart with the different ratios. I could not attach it for some reason, but email me if you want to see it.