There are no Mustang components used in the MkIV other than the engine and transmission. The car was designed to comply with US DOT safety requirements before Ford became vested in 1986.
There is no real MKI, II, III, that is a desination that evolved over time. Leafspring and coilspring denotes the cars in proper terms. The MkIV is a coilspring Cobra similar to the big block 427/428 cars and the AC 289 Sports. Yes, the nose is a little longer and the foot well are a tad bit larger. The MkIV is exceptionally well built. The aluminum body is a little thicker than the original Cobras but that is not were it ends. There is heat insulation riveted in place on the underside and firewall. There are inner fenders to protect against rock dents from inside the fender wells. The bumpers are telescopic mount. The wheels are 16". The dashboard is different. Headrest and seatbelts are included because of safety regulations. The shock absorbers are the same part number as used on the Ford GT40. The transmission is a Borg Warner five speed. The differential is Salisbury. The fuel fill is in the center. The paint is exceptional. The door latches are Rolls Royce. I have been along side Ron Finger's 1964 USRRC Competition. The similarities far outweigh the differences. If you want a MkIV that is truer to the original coilspring Cobras, Autokraft build 73 "Lightweight" versions with a short nose, hood scoop, original style dashboard and 15" wheels. Most of these cars were equipped with the Ford 340hp SVO engine. Please keep in mind that Autokraft built a few Cobras for Shelby. By the way, although the standard 302 engine was installed by the factory in most the MkIV's there were some 427's factory installed.
When I was in the market for a Cobra, I could not afford an original nor did I want fiberglass kit car. I found an ad for my car on Autotrader. I started doing some research and came across the AC Owners Club. On their forum there is a page dedicated to the MkIV's I engaged with them and found that the Club had a register for the MkIV's similar to the SAAC registry. Because Autokraft bought AC Cars Ltd from the Hurlock family, the car is a true AC with proper DNA. The source of all the original Cobras and unlike anything produce by any other company anywhere. As we know, there were CSX, COX and COB variants of the original Cobras. My car is an AK (Autokraft) not attempting to fool anyone into thinking it is an original Cobra. My car is AK1131 which is plated on the car in three locations and stamped into the frame. There was a total of 386 AK's built in England from 1978 to 1996 with an additional 73 AKL's (Lightweights) per the ACOC AC MkIV register. 459 total, less than half of the original Cobras produced. 183 built for the US market. It is what it is but it is a Cobra.