maybe he swithed the fenders at some time, and all 4x4s are k series execpt the newer ones they have c1500 offroad z71 and other things, it could of had 4.10s or 3.73, maybe even 3.08s, the only way to know is put the cover.
And the vin decoder just go to google type in chevy vin decoder and then find the one thats says this website as the host.or press the link.
http://www.67-72chevytrucks.com/Chevyear.html