Without reading this whole topic again, are you sure the carb is opening fully and the timing is going to full advance?
Take a look at the shift lock lever and the brass plate on the mag plate when you advance the throttle with the motor in gear and in neutral. There are steps on the brass plate and the lock lever contacts these to limit the motor speed depending on which position the shift lever is in. One step limits neutral speed and another might limit it while in reverse. In forward gear there will be no contact and this allows full throttle operation.
Here’s a ’56 mag plate but yours should be similar.
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