What gear for 6K in 1/4 mile?

I'm going to make guess that your car will run 105 mph in the 1/4 mile and et at 12.5.
With that you gear with 15 inch tire's you would need a 4.41 gear which they don't make so you would need a 4.30.
So rpm's would be 5825 and add a couple hundred rpm's for converter slip and you would get 6125 rpm's.