Actually, both shells are metal (I have two 4-6-0s, and can confirm they are metal; Spookshow's review indicates the 2-6-0 shell is also metal)... which would make modification to the inside of the shell more problematic.
I looked back and forth numerous times at the images in Spookshow's reviews of the two models, and can't really tell if the chassis would be interchangeable. The 4-6-0 chassis might have to be shortened some at the front, Also not sure if the steam pipes on the shell will align with the cylinders on the chassis... you will have to try it to find out. Unfortunately, Bachmann website lists NO spare parts for the 2-6-0... and I called them once to see if any were available, and they told me everything they had was on the website. So finding a 2-6-0 shell might require some patience...