Car is a strong runner, no doubt. 13.1 is on par with what most average driver stock cobras should run...or at least damn close. A set of 4.56 or 4.30 gears would put you solid mid 12s, definitely fast enough for any stock cobra.
Your trans actually has a taller (numerically lower) final drive than my 6-speed, even with the 56's youd be under 3k at 80. My 6th is a .63, your 5th is a .62...
http://www.csgnetwork.com/rpmcalc.html says youd only pull about 2900 even with the 56s. I assume its primarily a street car, and from what i can gather the consensus with the N/A 4v guys is 4.30s. 4.56s if you see the track quite a bit.
Might check this out...
http://www.svtperformance.com/forums...d.php?t=491834