mine also. it's a code "D" VIN, has metal tag and GVW sticker which both have VIN on it. plus, it still has the original motor. i believe it has only had 2 previous owners. i've seen alot of trucks and most still have orig motors. they weren't "beat on" like cars were. mostly work horses, esp in the 70's. so you'll most likely find high mileage original engine trucks.
course, that all depends on how original it is when it's been purchased.