That's a wide open statement covered in grey. No matter what the purpose it must function properly. Changing the cam and carb is like a heart transplant. It makes for a whole new engine... Most of the engines you see out there are basilcly a stock engine with a cam and carb change.
Forget about the size (diameter) of the converter. You want to know approx stall speed. While it would be nice to know the exact stall speed, unless you have a dyno sheet to send them to build your custom convertor it will be approx. Follow me..
My frst big block build I called dynamic, faxed them my dyno sheets and they built me a 9" converter that had a 3900 stall speed and it was pretty damn close. You could not tell this car had any special convertor in it under normal operating conditions.
My current car uses an Art Carr 3500 converter. It stalls a bit shy of that. You cannot tell that it had has any special convertor in it.
As said get timing on. I run 14/34 with no vac advance. Once the timing is set, adjust the carb.