The EFM's are similar to our Van-Wert, they generally have a smaller ash tub though. If I was going to do that I would leave the original base as is except add some rails to the bottom where the floor would be so you can continue to use it as intended. Get some custom made tubs for the new portion below the existing ash bin and build it its own door. The size(s) and configuration of the custom ash tubs would depend on the mount on space you have mostly on how high it is. 3 courses you could make 4 to fit in their nicely and they would fill up nicely.
The original ash bin will now be elevated and be easier to handle, the additional ash bins below can be used for a overflow. You could even skip the custom ash tubs but then you would have to shovel the excess which is going to make a lot of dust.
Better yet get a auger that goes outside the garbage truck can pull up too...
The only thing I really hate about coal is taking the ashes out.