well going off of JT's spreadsheet. a 4.10 gear in 4th gear @ 6500 RPM puts you right at 121 mph. so judging by your traps in your sig, that would be perfect for you and even give you some room. i am not sure how high you take it though.
What RPM are you shifting at? 03' Cobras don't go higher than 6500 and some of us shift earlier than that. 99/01 Cobra's are good to almost 7000rpm (unless you've run out of fuel system at that RPM). What does the car wiegh? You know RWHP so the only think that is variable outside of what I asked already is what size tire do you plan to run at the track (height is critical) and can we still assume you have the T45 w/ 1:1 4th gear ratio? On the surface yes 4:10s are probably good if you are sticking with 26" tall slicks. 4:30 might be better if you're willing to go w/ 28" tall slicks. BTW: If you want a larger audience of folks who have been where you are you should check out www.corral.net and check out the SOHC/DOCH forum or the Drag Racing forum.