what size stall should i run?

A poorly tuned car needs more converter. A well tuned car with a 360/484 cam should be able to work ok with a 2500sih converter. More is better.

A good 9.5 converter can drive around like a stock unit and flash hard, in the 3500+ range. They cost money to get a good one, but not much more than an off the shelf 10" converter that likely won't work as well.