Don't know what you saw, but it was not what you think it was. There is no hemi slant-6 -- not even as an engineering prototype or anything like that. The Australian-built Hemi-6 is an upright engine, not slanted and it is totally, completely different with no interchangeable parts (except valve lifters).
Got some pictures or a link to this mystery truck? Without them, I suspect what you saw was in fact a flathead six, and what you thought was the valve cover was in fact the top of the head.