First place I would look: how much initial spark advance are you running, and what carburetor?
Do you have a vacuum-controlled valve body?
Smaller diameter is one way to increase stall speed, but there are many other factors including turbine fin clearance, angle, and shape. A 10" converter will also act as a light-weight flywheel and give better throttle response but may be more stall than you like. You should be above your stall speed at your cruising MPH or gas mileage suffers.
What stall to use is partially dependent on your traction (8" is pointless with small tires), how hot the cam is (to idle cleanly), do you have power brakes, and when does serious torque begin.

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