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IIRC, almost all factory regular production transmissions from the 50's until the mid 60's have a 1:1 ratio for their high gear.
Unless the gear pitch is opposite (left or right handed) you may not need to change the speedometer cable gear at all.
Was the rear axle or its gear ratio changed?
As suggested... if it is currently working double check it before making changes.
Start by checking the odometer reading over a known distance, with gps or hiway mile markers.
The odometer is gear driven and more likely to be accurate than the tiny springs and spinning magnets in the speedometer needle mechanism.
It's very possible for the odometer to be accurate and the speedometer needle to read incorrectly. In that case it's the internal needle mechanism that needs cleaning and/or adjustment.
Generic example photo below showing the needle adjustment.