I had a MTH RK 2-8-0 and bought another RK 2-8-0 boiler shell from a forumite to replace the original. The mounting holes were the same, but the casting was different. I had to dremel away bits and pieces to get the frame to fit inside.
I have a Legacy GP9, Wabash 484. When I got it I tried looking it up on the internet and the only thing I could find was an older, conventional model. I have an Atlas GP9 and while they both look the same dimensionally outside, I know they're not the same inside. Mounting locations and thickness of the plastic used are 2 things I can think of that would cause concern about swapping around any shells.
I'd take the chassis to a train show and try finding a shell to do a test fit with, unless you're capable/comfortable of making alterations. At the very least I'd get some idea of the outside dimensions of the shell to make sure it fits.
