Shelby has stated he also very much considered an Oldsmobile engine, which he eventually did use in the Series 1. Along with AC he also considered different cars for the project, an agreement with Donald Healey could not be hammered out. The BRAND NEW Ford V8 was the perfect choice for this project, it weighed considerably less than a small block Chevy. The GM small block is a very heavy motor, but the Oldsmobile engine is heavier than that!
In my opinion an Austin Healey 3000 (poor front suspension and solid axle rear) combined with an Oldsmobile engine (700 plus pounds in the front) would have been a spectacular failure, for a road course in particular. There are numerous 'what if's' Shelby played out to achieve his concept. A concept all ready well known and in use since the early 50's at least. Big engine, small car, nothing new there. The DIFFERENCE was finding the right combination of car and power plant. The AC and Ford was just about the best combo available in the early 60's. For price, availability, performance and the general requirements to make a 'production car'.
Could have just as easily been an Aston Martin or some other European make with or without an American engine. WHY American engine? At that time in history the best bang for the buck, horse power, cost, etc. The new Ford small block was WAY ahead of the exisiting (but heavy) GM power plants available. Another consideration might well have been the ultra light 215 cube alloy V8 or perhaps an Offenhauser power plant.
You see, you gotta have the right package deal. Ford and AC, a good call, better than an old MG with a Ford flat head. What combination would YOU use in 1961 if YOU were doing such a project?
In 1969 my brothers and myself dropped a 327 into a Morris Minor! Didn't work out so well, virtually no brakes and no handling. But it looked cool and went like stink in a straight line! Shelby, we were not!
