Interesting - 1968 was the first year that Chevy went to spin on filters and the oil fill tube left the manifold and went to the valve covers. Trucks sometimes became the "red-headed step child" and GM used up existing stock of parts. It is not as uncommon as you think to have some parts interchanged on what would appear to be a factory assembled unit. I am personally aware of a brand new 1968 truck that the window sticker indicated was a 327, 2 barrel when it was in reality a quad. The new owner was a close family friend and she always thought her truck was a bit "snappier" than the '65 she had traded in!

Short of tearing into it - you may have a "mystery motor".. But hey, it runs good - so clean 'er up and let's see a few more pictures!

Regards,
Glenn