Agree with the drive ratio discussion. You should decide what is your main purpose for the cobra. If you are mostly racing, you should get the 0.82 5th gear and make your differential gear match your fastest desired speed in 5th gear. If you are going to be mostly on the road/highway, keep the 0.65 5th gear and live with the drop in power in 5th. If you are currently hitting redline in 4th and can't accelerate in 5th, carb tuning or gear changes (transmission or differential) is all you can do to help the poor acceleration in 5th.
But 10:1 ratio is also way too rich.
What is your AFR at cruise (1800 rpm on a flat road)? This is most likely your transition slot tuning.
What is your AFR at 3000 rpm on a flat road? This would be the primary main circuit.
Wat is your AFR at WOT? This would be the primary mains in combination with the secondary mains.
What happened when you changed your main jets? How much of a change did you make? My guess is that if you were at 12-12.5 at cruise and went to 10:1 at WOT you were rich all around and changing just the primary main jets would not help much. Likely you are still rich at WOT and likely normal to lean on mild acceleration.
If you can answer the above questions, we can try to help get your carb working more efficiently. It may not pull much harder at 140mph with your 5th gear being 0.68, but you will get much better gas mileage.
