Haven't done it all the way through, but have removed the various parts to do so. I have a 61 project that the PO removed the small and installed a big. I don't like the big, so intend to convert back. Its at the spot weld stage, where it waits motivation to finish.
Yes, there are more panels than just the filler at the sides. IIRC, it requires a different panel underneath, and some others. I would have to go study the cabs out back to be specific.
Going big window to small, u can probably make it work with just the fillers and some fab work. Small to large, not.
Plastic trucks suck...
Iron trucks rule...