I have a 0.68 OD gear Tremec with 3.73 rear gears. In 5th gear at 100 MPH, its
only reving 3000 RPM. The 0.68 gear is great for good gas mileage on a loooooong highway trip. I do not like to go over 65 MPH on highway (too much wind buffeting) so the RPM's in 5th gear are about 1800 RPM at that speed. It also cuts down on the noise, but I still use ear plugs on a long highway trip.
There are advantages and disadvantages of both overdrive ratios. If you intend top do a lot of highway driving, go with the 0.68. If very little highway driving or if you intend to race the Cobra, go with the 0.82. If you are going with a numerically low rear gear, definitely go with the 0.82.
Here is a good calculator. Play around with the various numbers to see what RPM's you will have at different speeds in various gears.
http://www.secondstrike.com/Technical/GearCalc.asp
At 7000 RPM, my Cobra should be going 221 MPH in 5th gear. Too bad its an airplane at 160 MPH.