Have you looked at...

the selections from Sylvan Scale Models. Perhaps a little rough, but they do cover 1950s grain trucks.   https://sylvanscalemodels.com/Grain%20Trucks.htm

Or searched on Magnuson 439-947 Grain Truck?

Also consider late 1940s trucks (post war production)  A good farmer would squeeze the last breath out of a piece of equipment, including trucks.  When I was custom cutting in 1970, one old farmer we cut for had a grain truck that was old enough to have a floor starter button.

Consider adapting a truck

Another possibility to consider is find a Classic Metal Works R-190 truck and remove the bed. Then you can build a  GCLaser grain bed and attach it to the truck. The grain bed is designed to fit the CMW R-190. This is the one I built a couple of months ago.

It is true that farmers use older trucks to haul grain.

Alloy Forms Chevy Truck

Have you looked at Alloy Forms Chevy trucks.  There are a couple on Ebay, I didn't look any further. 

Here's a picture of mine.

Wiseman has some 40's produce

Wiseman has some 40's produce and sand trucks that look like they might be suitable with minor modification. Also in a pinch I would think that any suitably canvas covered dump truck would suffice.

To me the single axle sand trucks and grain trucks look about the same. The box is not quite as high as the cab . The double axle trucks the box is higher .

Grain trucks

Thanks all.   I will try Sylvan and also Classic.  I had a grandfather in the late fiftys with a floor starter.  I had to get forward of the seat to reach the button.  Late fortys would work also.

Classic Metal Works. Not

Classic Metal Works. Not single axle but something different. I believe these are available with a 1954 Ford truck.

Check out photo.

Another Vote for Sylvan GC Laser and CMW

Agree with the above and actually just ordered another GC Laser bed to be put on a CMW chassis. Sylvan also makes a stand alone grain bed, but I prefer the GC product, as they have a realistic chute for the discharge.  I believe that CMW makes both a single axle and double axle R-190 truck.  I'm using version for a grain truck and the double axle for a logging truck.  Hopefully a project for this weekend

another way to do it

In addition to trucks, you can also have a tractor pull a grain wagon to the elevator.

Ken Vandevoort

living next to a seed corn/soybean field

grain elevator

Great idea.  I see that even today at rural elevators.