Carroll Shelby had a close relationship with his chassis supplier, AC Cars, but finding an engine proved a bit more difficult. For some context, the AC Ace whose chassis was used for the Cobra was originally powered by an inline engine supplied by Bristol, but in 1961 Bristol decided to cease production of its own engines in favor of license producing a Chrysler design. AC decided to switch suppliers, and sourced a Ford engine. However, when it was time for Shelby to start sourcing components for the Cobra in late 1961/early 1962, he decided to approach Chevrolet with a request for engines, but it declined to do so as it felt the new vehicle would compete with the Corvette. Ford agreed to supply him with an engine, and one was sent over to AC Cars from Ford of Britain. This information is from
here.
Thus, with relatively minimal PoDs it would be possible for the Cobra to have been powered by an engine from any of the Big Three of Detroit. Given the time frame, it might even have been possible for a Studebaker 289 cubic inch V-8 from the Avanti to have powered the Cobra. The Avanti engine was quite impressive, especially since it allowed the car to compete with the Corvette despite being a smaller and older design (information on the Studebaker 289 cubic inch V-8
here).
What would the implications have been if Shelby had found a different engine supplier? Could Shelby have gone on to establish a relationship with them on the scale of the historical one with Ford, or is that something that was only possible because of the vast amounts of money Ford was spending on its various racing programs for Formula, stock, and rally racing?