Shelby himself has stated his original idea was to use an Old's engine. When he went to GM he was 'following the money', he needed more than an engine, he needed a great of deal financial support. It was no different with Ford, just following the money. His timing was superb, Ford had just released it's new light weight V8, a 221 cube that would grow into the 302 we know today. But it could have been a Dodge enigne as far as that goes, he needed both the money and the motor!
There are many examples of an Austin Healey 3000 with a Chevy small block. There are several in Hawaii! They all have one thing in common, their a 'straight line' car, they simply don't handle very well. The AC suspension is far superior in that regard, though it to is 'antiquated' it is still head and shoulders above the Austin. I happen to own an Austin Healey 3000, have for years, I know it's limitations.
To suggest that a Chevy engine is OK in a replica Cobra because Shelby once considered the idea is a pretty lame argument. Shelby's vision could have been any number of combinations. Aston Martin with an Offenhauser? Healy with a Hemi? MG with a Caddillac? Masserati with an Olds? But in fact, it was an AC with a Ford. History has spoken!
283 GM, 575 pounds. 289 Ford, 440 pounds. Approximate numbers.