How to determine the needed stall speed?

I will parrot what others have said, call a reputable converter builder and give them ALL of the specs for your car AND it's intended usage and they will build you one to fit your needs.

They will want to know cam specs, rear gears, weight of the car, intended usage etc.

The converter will make or break your combo in the performance area. Remember you get what you pay for. I have known several people to buy an off the shelf converter, put it in and it would not function the proper way. Now they have to start all over again.