I'm imagining anything that isn't a road car can't do wide bodies. Racecars are already widened and are bound by rule books to be of certain dimensions. Open wheelers, to your point, would be difficult to imagine wide bodying. I have a hunch PD won't allow most VGT cars to be visually modified either. Also, with
how fiercely protective Ferrari have shown themselves to be of their image, I don't know if many of their cars can be disfigured like that, if at all. I'm going to guess that the 458 is the only one that can, taking on the shape of the 458 GT3.
As for engine swaps, it's hard to say. Again, Ferrari comes to mind. Most of the Gr.3 and Gr.4 racecars don't have specified engine displacements, much less engines. I'm guessing those will be stuck with whatever they have. Would be sick to swap in the 600HP gasoline engine from the Atenza Gr.3 into the road going, diesel Atenza though.