[Christ]

The .pdf vaguely says that while it's not technically illegal to swap between manufacturers, the cost may be prohibitive of such events.

Earlier in the .pdf, though, it specifically says that unless the resultant vehicle is in accordance with one of the certified configurations for the chassis, that the engine switch is not federally permissable.

This, of course, would mean that switching between manufacturers is not only cost prohibitive, potentially, but also still illegal.