I read that it is also possible to determine the engine size from numbers on the intake manifold. Numbers on mine are: 6E20, which I interpret as 1966, March 20. Also on the runner is: C6AE, and 425-G Another question for those with knowledge. I recall the 390 engine in the first 66 I had, had flat topped rocker covers. The engine in my 2nd car has rounded rocker covers, with "Powered by Ford" in raised/stamped letters. I know that is not a guarantee of engine size, but, if those covers were stock, it may help.