I thought I’d heard of this having happened many years ago. Fortunately for my odd combination and laziness, someone had already addressed the matter:
Yes, 1966 Ford Mustang GT350-Hs (for Hertz) were rented from the vehicle rental company before the engines were swapped out for nearly identical-looking engines before they were returned. That’s because the K-code 289 engines in the GT350s were rated at 271 horsepower as opposed to the lesser 225 horsepower A-code engines which were covertly smuggled into the cars. The K-code mills were popular for drag racing as well as road racing, and many left the factory making significantly more horsepower than their factory-rated 271 HP.
Today, one of the most valuable vintage Mustangs on the auction market is the GT350-H, often fetching hundreds of thousands of dollar$.
I’m guessing that an original, numbers-matching GT350-H will probably be worth more today than one of those sneakily-derived Frankenfords, but you never really know at a classic-car auction until the hammer comes down.