They had been specially invited to compete by the Le Mans organizers themselves. As might be expected, they were blazingly fast while they lasted, but weren't designed to endure like the 24 hour cars.
NASCAR stockers can go 600 miles with some reliability. But not at Riverside, The Glen or Road America. But 4 times that is a bit of a STRETCH! Ditto at our Daytona 24 Hours.. Been TRIED! But with little to show for it....
How long does a modern 2019 NASCAR engine run in 24 hours Le Mans racing? I rather race my NASCAR powered Crown Victoria in Forza 7 to find out how that might happen in real life. I go for NASCAR powered street cars as options.