Here are some thoughts on dealing with cooling your motor in a 37-38 Willys......
Radiator-Usually a downflow radiator works best with a large upper and lower tanks giving good coolant capacity....and the tallest and widest radiator which will fit ahead of your engine between the inner fender wells with as much exposed to air flow through the hood louvers. The radiator needs an inside the hood air dam such as the stock Willys hoods came with to help divert the incoming air into the radiator not allowing the air to by pass the radiator. The stock air dam might work but more than likely you may need to build a custom air dam fashioned after the stock air dam which will work with the radiator you are going with in your particular car. This is very important. Examples below.....and here is a Bill Jansen diagram of how the air dam works regarding air flow through your hood louvers below........You might also like to review Bill Jansen's 37-38 Willys Cooling Page as well for more background and answers.
Willys Cooling Diagram-Bill Jansen
Electric Fans-Always best to use pulling electric fans if there is room between the radiator and the engine. If not, then pushing fans is the alternative. Largest possible electric fan or fans to push air through is very important. If it is a pulling fan, a full or partial shroud will only make the cooling effort even more efficient.
Air Flow-Whenever air get through the louvers in the hood after flowing through your radiator, it needs to exit the engine compartment otherwise it is like a dam and no matter how good your fan and entry is for incoming air, there will be no air flow for removal of the heat from your raditor. Some of your options are to open up the inner fender wells on both sides in the areas of the stock louvers....removing as much sheet metal as you can but not compromising the integrity of the inner fenderwells for supporting your steel sheet metal front end. Another simple option is to reverse the opening of your hood's fresh air scoop which would allow air inside the engine compart free passage out of the compartment. See the hood scoop on the pickup to the right below. Scoop is reversed and stays open all the time for this reason.
Willys Hood Inside Air Dam-Came on all the Willys cars and trucks from 37-42....The air dam was designed and installed to fit down onto the top of the radiator which the stock radiators had side air dams as well so that air flowing into the engine via the louvers of the 37-38 hoods would be diverted inside into the radiator and not around it. Very important for making your cooling system work is to make use of the air dam concept in your cooling system design.
Front End Scoop-Numbers of Willys owners make custom under the front sheet metal air scoops which catch fresh air and divert it up and into the front of the radiator in the engine compartment. The other option is to cut a slight opening in the lower sheet metal below the old Hand Crank Hole in the shape of a "slit" but a little larger as this Willys delivery owner did on the right below.
Small Grills embedded into the lower Front Sheet Metal-Some very clever fellows in the UK embedded small grills (engine hood side grills from an English Popular or Anglia) into the front sheet metal on either side of the Hand Crank Hole to allow direct fresh air entry into the engine compartment and to the radiator. Some others did the same thing by installing actual louvers into the stock sheet metal on the lower front section on both sides of the Hand Crank Hole.
Removing some of the Hood Louvers-some guys have actually removed some of the hood loovers leaving larger openings in the stock hood which allows more air flow into the engine compartment as in the pickup pictured below.
Auxillary Cooling-Ray Fields in LA cleverly used two long finned alum coolers, 36 to 42 inches in length hung up on the running board L Frames where they cannot be seen plumbing them into the engine heater circuit allowing another means to circulate the hot water and expose it to the cool air on the underside of the car.
I used Ray' Field's idea to install two small and shorter finned alum coolers right under and in front of the front stock crossmember which were plumbed into the engine heater circuit. The finned coolers pictured below are the same ones mentioned in the above suggest for inside the running boards, exept those are much longer than the ones below.
For Pickups-Rear Radiator Option-There are numbers of Willys pickups running rear radiators, including Bob Hudson's in Bakersfield, my own and others. They use normal steel pipe (Thickwall) between the engine and the rear radiator normally of the same diameter as the engine outlets and radiator inlets so that short rubber water hose connectors can be found in your local auto parts store. The steel pipe should have typical radiator drains installed in the low section on each side underneath to make it easier to drain when engine or cooling system maintenance is required. The rear radiator needs to be installed such that the radiator opening is higher than your engine to help when bleeding the system of air bubbles. the egine water pump will handle this extended system just fine. No additional water pump is required. Electric fans mounted to pull air through the radiator is the way to go coupled with a full shroud to pull maximum air through the radiator. It can be mounted behind the cab or even better at the back of the bed where it can catch air flowing over the cab and down through the tailgate area. Pictured below is the custom Ron Davis Racing double pass alum radiator with full shroud and two large electric pulling fans which I use in my willys pickup. the bottom picture shows the pipe hung underneather in the chassis with a drain welded into it for draining the syste. I have used this system now for 12 years and it works well.