Tough one to answer snkbte because there are so many variables.
Firstly, rear end ratio and tyre sizes have a huge bearing on the decision. As does the purpose of use....street racing, drags, highway cruising.
Factor in all these and do some sums. Here's a link that might help with the sums.
http://www.368s.com/tools_rpm.phtml
A trap many on here have fallen into has been to not do the sums properly. Many cobra owners are using 5 speed transmissions but because of their diff ratios find they have a totally useless first or a non-usuable 5th gear.
If one is using a 5 speed...then why not use all the 5 gears available. Why have a 5th gear that drops revs so low the motor is "off the cam" at a highway cruising speed.
Tremec have released a new line of transmissions, the 500 and 600. Increased power rating but more important..better gearing.