Well there is something to be said for that E-Rod GM engine. The first being, loose the coils on the valve covers, they look hideous!
Interesting thought that Shelby might have gone big block in response to a possible 396 Vette. I've never heard that, one thing is pretty clear though. By 1964 they were certainly looking for ONE thing to happen with the Cobra, a higher top speed. Maybe a big block would provide the needed punch? The small block was maxed out. The ultimate solution turned out to be the Coupe.
I bet it was GM's paranoia over the Cobra's performance that lead to a big block Vette (desperate men do desperate things).