The car was built by Dick Dean and George Barris. Here's a photo of the car at Barris shop (notice the lower portion of the left front fender):
Here's a closeup of the lower portion of the left front fender:
The car was offered for sale at the same May 12, 2005, auction by Bonham's at the Petersen Museum where Barris tried to sell the fake American Graffiti '55 Chevy. In the auction announcement, the Barris Grease Lightning replica car was described as "Grease Lightning, the 1964 (sic) Ford designed after the Ford seen in Grease, this car having shared a stage with Olivia Newton John during a recent concert". Notice that the catalog description did not claim it was the real, screen-used movie car. (Luckily I printed this bulletin out back in 2005 as it has since been removed from the Bonham's website. I've posted a couple of scans from my printout)
It would appear that the Petersen Museum purchased the car knowing it was "designed after" and starting displaying it as the real Grease Lightning car. The display board for the car states
“This 1946 Ford was customized for the 1978 musical Grease, the biggest grossing movie musical of all time. Featured in the film’s final scene in which Danny (John Travolta) and Sandy (Olivia Newton-John) ride off into the clouds, the car was transformed from a stock two-door sedan into a convertible by renowned Hollywood customizer George Barris. Its enormous fins, clear plastic hood, and wild paint scheme distinguish it as one of the most striking fantasy vehicles ever featured in a movie about teenage life during the 1950s.”
Notice again the lower portion of the left front fender. At this point, I would place the label of fake on the car since the museum is displaying it as the screen-used car from Grease. Fortunately though, as with many fake movie and TV cars, those who try to build a replica of the real car miss out on many of the correct details. Compare the lines of the Barris-built fake Grease Lightning and the real car from the movie:
Notice that the point of the rear fender fins is much lower on the fake car, notice that the radius rear wheelwell is smaller on the fake car (same mistake Barris made on the fake American Graffiti '55 Chevy) notice that the rear fender swoop addition starts much lower on the fake, and also notice the difference in fit at the rear of the clear plexiglass hood on the fake when compared to the movie car. The accent stripes are much different also.
Then there's a noticeable difference in the front ends of the cars. Here's the fake car's front end:
And here's the front end of the real movie car:
Look at the difference in not only the shape of the grill, but the number of bars in the grill itself. Also, notice the difference in how the bumpers are shaped.
Next, we can examine how the headlight buckets were filled in. Here's the Barris-built fake:
And here's the real movie car:
Looks like Barris went with the bulbous look instead of the flat panel where the headlights used to be. Also notice the cut of the fender where it joins the grill.
Here's another view of the front fenders. First, the Barris-built fake:
And now the real movie car:
Here's another detail that Barris missed. Look at the split seat in the Barris-built car and compare it to the obvious bench seat in the real movie car. First, the Barris-built fake:
And now the real movie car:
Looks like Barris also missed the spotted dash cover as well.
I later ran across this post by Eddie Paul on the Ol' Skool Rods website: