Tony:
Spent a lot of time with gear ration calculators before I decided on my final setup. Try
http://www.368s.com/tools_rpm.phtml for an easy to use calculator. Just plug in the numbers for transmission, rear end ratios, your planned tire size and it will give you the theoretical values for speed at rpm in all the gears.
Heed the advice about going with too low a rear end ratio. I'm running about 450 HP with 315/35/17 tires and a 3.07 rear on a TKO600 w/ 0.82 fifth and can spin the tires at will in 1st or 2nd and run 2300 rpm @ 70MPH on the freeway.
Very tractable in town and a real pleasure to drive. One of the more common mistakes people make is to compare the Cobra to other cars and what they're running for final drives. Your Cobra is going to be LIGHT and that makes a BIG difference in what will be useable in the end.
DonC