The southern yellow pine (SYP) used in the 50s-60s might not have been the old growth SYP "heart pine", which is much denser. Most of that dense old growth pine was harvested in the 19th century. Any old growth pine in the mid-20th century would have been very expensive compared to standard-grade southern yellow pine.
Don't you think that standard grade SYP was used for truck beds (these were just work vehicles)? I do not know, but I know everyone says that "heart pine" was used.
