Sorry, I don't buy it. If you look at http://www.racingicons.com/gt, you'll see numerous restored GT40s. Both late MkIs and all MKIIs had the quad light set up, long before the '67 MkIV. Our GTs are based on the wide body MkIs that featured the single and larger pod lamp/side, but I suspect as the originals were primarily race cars, whatever got slapped on them was what was laying around at the time. I imagine Ronnie Spain could enlighted us with the real answer.
And I call "Foul-Urban Legend" on the story. I do NOT know how the Corvair tail lamps ended up on te MK II and then the MK IV BUT I seriously doubt the "down to the parts store with a scale" story.
The "J" car used a 1965 Thunderbird tail lamp assembly (without the turn signal sequncer system the T-Bird used) The plus side: Distinctive and and a Ford parts bin item (lens P/N C5SZ-13450-A). The down side: The tail light bucket weighs around 4 plus pounds of die cast zinc and used three 1157 bulbs. Now on the "J" car they might have moulded the bucket into the rear clip. Having never seen "J" tail in person, I can't comment. However, the T-Bird tail lamp would not work on the MK IV as there was not sufficent horizontal space for the lens which is around 15" horizontal width.
So we need some smaller, round light units....if indeed they sent someone to the parts store they would NOT have been able to find these. To wit:
1.) The lenses would only be sold at a GM dealer as an original "Guide" brand item. A generic parts store WOULD likely have the aftermarket lens as the "Glo-Brite" brand. Most volume tail light lenses were offered by Glo-Brite via independant parts stores (NAPA, etc.) In fact the Superformance GT40 MK II uses the Glo-Brites as the "Guide" reproductions are multiple times more expensive. Note to SPF owners, you need not source a replacement lens from South Africa....)
2.) A generic parts store would not have had the bucket/reflector/socket/wire assembly as they were not serviced aftermarket then. Only a GM dealer would/could have had these in 1965/66/67. Today they are available as reproduction for Corvair restoration (is that an oxymoron?) but would not have been found on the shelf at Autozone/O'Reilly/NAPA/Kragen/Bubba's House 'O Parts in those days.
Someone could tell them over at the Ford GT forum where Jalopnik picked up this story....and that is just what it is, a story. As Mulder said, the truth is out there, that story just isn't it. Perhaps this another Murdoch plot!