Use an online calculator (a good one) to determine max RPM and trap speed for the type of racing you want to do.

Then, use the numerically lowest gear that will provide that final speed at max RPM with converter slip factored in.
You may wish to go a bit lower (numerically), the more power you have, as it will give some headroom for more power later or other tire choices and future additional slip.

Gearing multiplies torque, which is what we can never get enough of. But gearing also shortens gears and causes tires to spin and limits top speed- all of which may slow the vehicle down.

Depending on the type of racing, of course. In the 1/8 mile a 5:1 gear ratio might be fine. But in the 1-mile racing you would want like 2.5:1 or something.
So this is a racing-dependent type of question. And if not racing (say, daily driving) then it becomes an economy type of question.