Actually the "Mark IV" handle comes from the AC Cobra MK IV but I am pretty familiar with the GT 40 MK IV.
Other stuff to look at: between Sebring and LeMans several other changes took place with the cars, many dependant upon wether they were Shelby entries or Holman-Moody entries. Front brake openings in the nose were covered over with aluminum or glassed in, tail was hinged from the "B" pillar bulkhead rather than the rear of the car (failure of Shelby to change this caused a real problem with J6, McLaren lost the tail on the course and had to pick it up and return it to the pits for lenghthy repairs. Lots of racer's tape fixed it. But the officials required the tail to be "openable" so latches were made with belts including Shelby's EXPENSIVE alligator belt)
I too, built all the IMC kits in the '60's. I loved the Ford GT and still have several but in retrospect they were flawed kits. Several scale liberties were taken with them. But then again how many kits in 1965 had thay level of detail and operating features? I have all the versions, MK I, the IMC MK II (also released by AMT, Testors and Union over the years. The "J" car and of course the MK IVs. Plus the MPC "J" car and the MK IV. The re-release MPC MK IV can still be found on occasion at a decent price.
There are some 1/24 scale multi-media kits of the MK IV available that are very nice including an "end of race" weathered version.
Rick