Only story I "for sure" know about of an "unauthorized swap" was in my home town. An extended family had been in the car business for years. My Godfather worked for them for awhile as a salesman. He told me once joking, that "they carried every line that went broke. Kaiser Fraizier, Stude, Nash, and AMC"
(At this time they were Chrysler/ Datsun)
Anyhow they had a nice used Chev elCamino that hand a 365hp 327. THE BOYS ACTUALLY managed to pirate that 327 and Muncie out of the 'mino one weekend when the boss/ father/ uncle was gone, and swapped it into a 55 Chev, AND swapped the oily smokey 265/ 3 speed BACK into the 'mino. I HAVE NO CLUE how they handled the engine / trans mounts, if you are aware, the late model has engine block side mounts and a transmission tail mount. The '55 had FRONT block mounts and BELLhousing mounts and NO transmission tail mount.
But there it sat- back on the lot--before the boss got back on Monday!!!