I think that my adjustment was about 18% (3.55 rear conversion and ~31" diameter radial tires).

Finding a correct combination of drive/driven speedometer gears for an original 1941 4-speed transmission seems quite unlikely.

If the company listed by The Green Farmer can provide an adapter, its prices look to be very good (#54RA2 might have correct cable fittings). Give them a call. Ask if they can make an adapter for a specific % correction.