all you need to know right here bro... If you buy an automatic somewhere, they should now how big of a converter and what kind of stall would be good for you
When you take off real hard the converter will stall at the rated stall RPM provided you have enough torque to support it, if your torque is low it will stall lower than spec and if your torque is higher it will stall at a higher RPM. So it lets the motor work a specific RPM and torque to allow maximum acceleration that is very repeatable. So for example my 2500 RPM stall converter starts off at 2500 RPM from the line which is right in the sweet spot for my cam Crane 2030, Edelbrock performer intake and heads. This makes for a very quick launch especially with Nitto 555r drag radials.
Your cam and engine combination ususally dictate your best converter stall RPM.