Jay has very good points, and et me play on here too. BTW I answered similarly on your engine post
1 - You need to look at 1st gear compound ratio AND 5th gear
2 - A decent rule of thumb is 10-12:1 in 1st gear (depending on the weight of the vehicle and cam, also called starting line ratio) and 2.6-3.0 in 5th, for a street vehicle, more dependent on torque cruve of the engine
3 - I like to look at it this way "You have to think about getting 1st gear right, then 5th you need to get close"
4 - With that in mind, cam to the 1st through 4th gear use, and look at 5th, more than likely it's fine, but if not, lean back the other way or play with intake centerline
So here is an example, my 489 FE was a 3.70/TKO-600 combo, 28 inch tall tire, 10.62:1 SLR, 2.36:1 5th gear. That setup was very fast, no concern with torque even with a car MUCH heavier than yours (70 Mustang with A/C, PS, etc) However, the 2.36:1 was too tall on the highway. It didn't cause problems, but I found myself driving real fast when I was with a group. I changed to 4.10s, and my mileage went up, acceleration went up, and 5th gear was in a more friendly speed and still can go faster than you'd imagine and old Mustang should go. FYI, my SLR is now 11.76 and 5th is 2.63
Remember, an RR means road racing, not a great match for a heavy car IMHO, I like to go deep 1st and a non-RR tranny. It allows you to be a maniac, then drop into 5th and drive away.
However, a Cobra is different. It is light, and too much 1st gear will likely take away from performance. So an RR allows you to controllably launch a very light car with a lot less rear gear, and then you can still have a usable 5th.
I would expect that the lowest gear you'd want is a 3.73 and you'd likely be better with a 3.50 gear. (10.04:1 SLR, 2.87 5th with the 3.50) Now, the 3.73 is not TOO deep, but, with the light Cobra, you don't need more 1st, and that same weight affects what it will like in 5th, so IMHO, I'd take advantage of the light car and get below 3:1 in 5th. However, it's sort of splitting hairs now, because 3.50-3.73, really is all good, either one will run like a scalded ape.