Lionel #456 & 456R Coal Ramps

 Or, you could re-fit the hoppers with Post-war trucks.

That's what I did.

What's the difference in the trucks ?

I know the only postwar hooper that works on the original 456 is the N&W. 

What's the difference in the trucks ?

The important difference is the location of the uncoupling armature (part that pulls down to release the knuckle)

To clarify the truck difference:

Postwar 3456 hoppers have a standard metal truck, with a 480-xx coupler plate. The uncoupling armature is approximately under the outer pair of wheels.
The Modern era operating hopper that I have came with 9550-xx die cast sprung trucks. The couplers on these trucks have an armature that is similar to what postwar Lionel used on their plastic trucks, in that the uncoupling armature is mounted to the truck body/ coupler shaft, and a piece hangs down, near the knuckle, well past the outer pair of wheels, for the uncoupling magnet to attract.

I don't know what trucks were used on any later issues.