The Donut account is close. The decision to use the 427 came in Dearborn in the fall of '64. They didn't approach Shelby until after the disaster at Nassau in December. The aero figure is very close (76 hp) to Ford's own records, but the small block cars were already using the same Weber carb 289 as the FIA Cobras. They took two 427s to LeMans in '65 and both failed to finish. Ford realized they needed a NASA type effort and every pound of weight possible was carved off the 7 Litre while at the same time it and a new transaxle Ford built in house using the gear cluster from their NASCAR transmission were run to destruction at a winning pace on a dyno. The failed parts would be re-engineered and testing begun again. When they went to LeMans in '66 Ford had the most proven power train in the race. Brakes were still an issue but they had a quick change brake arrangement and could change them in minutes. All of the weight shaved off the engine was put back in the car in the form of safety equipment. After Walt Hansgen's death in testing H II wouldn't hear otherwise. The MKIV that won in '67 was a whole new car using the proven power train. It's development cost us Ken Miles. Along side Dan Gurney probably one of the greatest driver/engineers who ever walked the planet. Put Duntov in that group too. The '68 and '69 wins were by Wyer Engineering using his Mirage/GT40 variant rebodied back to close to MKII shape but with his suspension improvements and the best small blocks, 302s, Ford knew how to build. Oddly enough the same chassis won both years. Then it was over. The French coerced the FIA to do away with the large engined cars and we all know what happened. Porsche read the rules carefully, built a winner, then mass produced it to stymie the French for a few more years.
From GM's perspective the moment they really stepped on their di..um. You know. Was when Ole Shel came to them and asked for a couple of their new aluminum V8's to put in his sports car he was going to build. Luckily for Ole Shel they turned him down. The Cobra would have just been a better looking Daimler SP250 with that POS under the hood.