It looks like an MG engine to me or at least something BMC related, but I'm unsure, I would get a second opinion. I understand you're probably not familiar with Holden engines, but I can assure you, in this case it isn't a Holden engine. The Holden Grey motors (they were painted grey) went from 1948-63, and they used the same basic engine configuration for the 6 cylinders up to the mid 80's.