They were based upon the Hatchback. This can be verified by their VIN, indicating the hatchback configuration.
Here's How:
The hatch was removed, prior to the "B" and "C"pillars being cut, at their base. Then the "B" & "C" pillar bases were "finished" ~ Lastly, that 99% of the people do not know.. is, as you know, the two most outer tips (or "points" ) of the hatch, form the Capri's rear most fender tips. With the hatch gone, there was no fender tips at the rear. Rear fender tips were made (not sure...I think out of lightweight steel) and were welded in place forming new rear fender tips. These pieces are longer than you'd think and come forward from the top of the taillight area towards the door crack where the door opens. I'd say they are in overall length 2 to 2 & a half feet long. In some cars, at this age, along the edge of these pieces rust is forming.
I am pretty sure that Hank has several of these, sould anyone get hit in the rear, but, on these Capri converts, getting a substantial rear end hit pretty much makes the car a total. Actually the same goes for a Mustang McLaren, but there are not as many pieces on them, in the rear, as they did use the Coupe.
Now, the heresay...
I was once told by an ascMcLaren employee that the 50 - 1984 Mercury Capri Converts were made "custom" on line for ASC by using the entire rear clip of a Mustang Coupe ON a Capri, FOR the convenience of ASC workers.
I was later told by 3 people that the guy who told me that was smoking some very funny stuff..... I still think that guy was thinking out loud, of what might have been a good idea, but in actuallity didn't happen.
Good question. I was always surprised that nobody ever asked it.
The Mustangs were built under a D32 code (I think that's the right code) wherein alot of things under a normal build were altered. But, non as severe as the Capri 'Verts.
Here's the link to the "Here's How" on the 1987-1990 McLaren Mustang build:-
http://www.ascmclaren.org/phpBB/viewtopic.php?t=20