LMH - True regarding the 427 Cobra. The original small block Cobras, the ones that "started it all", were all AC. AC had already made the mods for a SBF (namely the Ford Zephyr engine). They reworked the front end, put in a stronger rear end, added outboard brakes and moved the steering box to clear the V8. What AC exported to Shelby was a completed, trimmed and painted car without motor & trans for Shelby to put in.
REAL_1 - As for Shelby, no question he took AC's recipe of a V8 powered roadster and eventually made it work in the states, but the "I got there first" award goes to AC Cars with the 24 hours of Le Mans finish in '63. That proved the racing viability of the car by an AC Cars Ltd team, not Shelby.
The question the OP asked is why "AC gets 95% of the credit" for the Cobra? Above are the reasons why.
Sure, a diatribe into the racing history of Shelby after the AC Cars Le Mans performance, and 2 years later with a 2nd gen Shelby Cobra, may deflect some of the attention away from AC, but OT to the subject of this thread. From an historical standpoint, your comments are a lame diversion transparent to most who see the irrelevance to the OP's question. An analogy to your argument would be giving William Tell credit for the invention of the crossbow. WikipediA has it right on the AC Cobra, choke on it
