It is possible that your truck came from the factory without smog. Heavy trucks had no smog gear other than a PCV system. I have a 77 F-100 and it has EGR, CC BUT NO SMOG PUMP. The F-150 and larger didn't even have that stuff. Back then they were considered heavy trucks and were smog exempt. I know cause I stuck a 460 in my F-100 and because that engine was not an option, I would have had to conform to 77 pasenger car specs. If it was an F-150, no smog equipment required. I had to pull the 460 and put my 6 back in in order to comply with the new 75 and older law. I say, take it to a smog referee and let them decide.