I would think where you'll be racing most would be a factor.
I.E> 1/4 verses 1/8?
If the 1/8 I would start out with a 513 and see what the motor likes, maybe up to a 538 or down to a 488 remember the more motor you have the less gear you need, but likewise the less gear you have the harder on parts it will be.
In the 1/4 I'd start out with a 430, or 456 and go from there to see what the motor likes and where the rpms are in the traps. But what do I know.
Goodluck, Charles