Converter size?

Generally, a smaller converter with the same stall speed will be tighter/more efficient.
To exagerate: a 11" converter loosened up enough to stall 4000, or a 9 inch stalling the same 4000. I'd bet the 9" would be 5 mph faster in the 1/4. And a lot more drivable.
(The first would be a GER special, lol).
Of course, the skill and quality of the builder enters into the equation.