Now who would dare put 4000 miles on an inline 6 truck? Out of curiousity, would there be a rebuild tag located somewhere? I have a newer rebuilt engine that had a tag riveted to the block. Anyway, it wouldn't surprise me if your engine was started on the assembly line when the 1960 heads were being unpackaged. I would look for the date codes such as 0A13 or something like that. On my rebuilt I have different years' heads and the date codes verified that it was rebuilt because of that.