[Bob Carriere]

Hi Gareth

A quick search of the web should produce suppliers of generic aftermarket overdive systems....but most will be USA located.... and they are expansive.

Remember that overdrive means overdriving other components as well..... brakes must be top notch, gears/bearings need to be in good shape and will make more noise, steering components....tierods...kingpins.... radiator capacity.....etc.....

I would suggest you install a temporary tachometer and record RPM at various speeds and see if the engine is really screaming at 40mph.... you might still be within range albeit at the top end.......

Consider the expanse of the conversion versus the times that you will actually be cruising at 40 MPH and see if it is worth it.

Rear axle gear ration probably can be changed but the savings in rpm may not be as great as an overdrive system which usually saves about 25 to 33%.
Find alternate gearing that will fit the existing axle casing may be difficult in the UK....... somewhat easier in the USA/Canada with more old vehicles around......... I would try to search Ford shop manual-parts manuals to see what gear ratio was available in the 1937-39 eara for the civilian market...... 1 1/2 to 3 ton range......

The other choice is to enjoy the scenery while driving slower.....

Keep us posted of your discoveries......

Bob C.