There are two basic issues which I think you are asking about. The Truck and the Motor. Probably need to decode separately.
We will have to be careful here with terms. VIN is usually a "Vehicle Identification Number" on a VIN plate on the cab and the same number on the Title. So is this what you are saying with C8Z143718 ? or what?
Sometimes there are partial VIN's on motors and other components of the truck. The frame will have a partial VIN stamped near steering box. The motor has casting numbers and Serial numbers. The casting numbers are cast into the head and the block and can help identify the motor. As Jerry says, there is a serial number usually on a machined pad in front of the passenger head. It is stamped into the metal and will need cleaned off to see. Don't sand it or grind it. This number will pin down the motor as to size, etc.
So far C8Z143718 does not make sense or conform to what I've found. So tell us what your definition of VIN is and where you see this number and clean and re read it again. If it is a cast or stamped number, clean it off and bee sure an "8" is not an "E" or "B" and so forth.
As you have discovered, the block was used from about 68/69 to about 1975 or ?. Good block. Could be the Truck version 327. Could be a 1968 car (C8) motor but need to know more info. Just curious: Is there a oil filler pipe sticking up in front of motor or do you fill thru the valve cover?
