I have a 3:73 rear gear with the 0.68 fifth gear and 15 inch Goodboot tires. Very mild 351W with about 425 HP at crank. Idles nicely at 900 RPM. Fifth gear is only useful above 70 MPH. So unless you intend to do a LOT of "
illegal" high speed highway driving, go with the 0.82 ratio 5th gear.
The only benefit of the 0.68 is when doing a very long highway cruise at 70 to 75 MPH ... it gets great gas mileage.
At 100 MPH, my motor is only turning at 3000 RPM. That would be great if I lived by the Bonneville salt flats.
Here are some nice "calculators" to play around with ....
http://www.cobranet.com/calcs.htm