Another option is to use a 53/54 senior rear end/axle. You would go from a 4.09 to a 3.54.

The track width and bolt pattern are the same. The swap just requires moving/changing the spring tabs on the axle. Speak to Flackmaster. He has done the swap before.

This is what I am eyeing for the Jackard.

I also have an overdrive in the cardan shaft. Best decision I've ever made. 2 liters less fuel consumption, 25 percent reduction in engine speed. The car is now really suitable for long journeys.

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Leaking, as in coolant getting into the cylinders? If that was the case, you should've probably seen other issues

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What do you mean - not sure? So you got the car sight-unseen, and did not even ask about the paint?

Okay, I just looked at the photos... I would not touch a nice car like that with any mods (even the ones that no one can see, because you'll know what you did ). An original car is only original once. Very easy to screw this one up, impossible to undo. All of them "modifiers" are always looking for an unrestored original, to do their dirty deeds with. While no one usually wants a modified vehicle, unless you did it yourself and can no longer sell it for the same reason.
Old cars are old cars, if you want one with overdrive, you can always get a newer model

Correct. A heavy duty fan was introduced for 38 junior models to improve air flow in low speed cooling and was later authorized to be fitted on several other earlier and later models. A later second heavy duty version fan was also introduced for other models the first fan wouldn't fit. The bulletins mentioned the fans would not help problems with high speed cooling. Trying to correctly identify and find one of those slightly different heavy duty fans today would probably not be easy.

Overdrive is one of the best things they've ever come up with. I wish there was a unit that would bolt into the 35-36-37 cars with no frame mods and retain the floor shift. OD offers great in-town performance and excellent results on the open road, and you get a "passing gear". On a coupe or 2 door car, I don't think you'd need to change the 4.09 gears to the factory issued OD lower axle gears. It might be a good idea on one of the long wheelbase cars or for a lot of mountain country driving.

The stock fan on the '37 120 has a rather low pitch angle and no shroud. I don't think it turns very fast relative to the crankshaft speed. The low pitch and low speed were done in order to keep noise down and support good fuel economy. The '38 or '39 120s had "cooling tunnels" added to front inner fender panels to exhaust hot air from under the hood. Refer to the literature library on this site for detailed information. I believe that 2 different fans were used on the 120s, with the later one having more pitch.

Just my two cents but OD is nice to have and if the budget allows, I would keep an eye out for a 48-50 junior transmission with R11 OD and pick it and any of the needed auxiliary items up particularly if you find them at a good price and even if you need to store things somewhere long term. With your assignments, maybe some family members would donate a few feet in their garage so you could have things shipped there and wouldn't need to try and cart things around. In 5 years or whenever you start your longer trips those items won't be any cheaper or I doubt in any less demand but you will have a better option if you have the parts..

If situations change or you decide not to go ahead with the project then you can always sell again and probably make a few bucks because you would hopefully have a more complete setup or maybe a full ready to install package by that time.

Another option if you want to have an OD but keep the car almost original is to remove the driveshaft and set it aside for reverting back to stock then install something like a Gear Vendors OD unit in its place. That unit could be supported by an easily removed structure bolted to the frame rails and would probably require little more than a few holes in the frame and new custom made sections of driveshaft in the way of mods to the car. As I recall, several years ago one of those units went into an early 30s model Packard. Some photos were posted here on this forum but may have been lost with the server upgrade. All the progress photos and details were on a private website which I don't believe is accessible anymore.

Many thanks for all the information. I can drive for hours on the highway at up to 100 km/h (with overdrive) and the temperature is stable in the middle of 100-180. Only without or with little airstream does it quickly go up to 180 and more. The cooling system is ok. Constantly restarting is not a solution either. Yes, the question is where to mount the fan.

My 120 has an electric fan ahead of the radiator. It is mounted very close to the radiator and above the center of the radiator core. It draws a lot of current and moves a lot of air. However, when it is not operating, and the car is moving down the highway, it restricts airflow through the radiator. Mounting a fan in the "well" just behind the grille and below the radiator core might be a good idea. The fan would need to be tilted somewhat to direct air backward and upward toward the radiator.