how does someone go about figuring what number is right for a particular engine setup?

an example being what is the difference between say a 2500 or a 3500 stall?

 

call ATI or another well-known converter company. ask them what is best for your application. be ready to answer some questions though. things like cam lift and duration. intake style, rear gear ratio. compression ratio. it's best to get the custom application by ATI or another custom converter company because the multi-purpose converters don't normally last but about 2-5 years. my father's ATI has lasted roughly 15 years (that's all hard, footbrake style racing too) and i will be purchasing mine for the berly here within the next 6-8 months.