I have a 79 1/2 ton 4x4 with a 351M. Emissions has been a pain. My dad bought the tuck new and instantly removed the stock exhaust with the cat and put on dual exhaust so that he could run regular gas which was cheaper than unleaded back then. Fortunantly he kept all of the rest of the emission components intact. So when I moved to Arizona 7 years ago I put two cats in my dual exhaust and added tail pipes so that I could pass emissions.

I think your sitituation might be a little different though. I believe they determine what you need to pass as far as emissions go by the vin number. And I thought that in 79 you could only get a 351M or 400 in the 4x4 and that the crew cabs where only available in 2wd. Also I thought that in 79 only the 1/2 tons only had the emissions stuff and not the 3/4 tons. You may want to start by decoding the vin and determining exactly what motor you are supossed to be trying to meet the emissions for.

I plan on doing some engine work one of these days so when I went through emissions in January I asked the guy what all I needed to have. Fortunantly here in AZ it is easier than California. In California everything must be stock even the air cleaner with heat riser. Here for my truck I need cats, PCV valve, air pump, engine vent into the air cleaner. EGR valve and evaporative system. I may have missed something. And also you must pass the sniffer test with the values determined by your vin number.

I hope this rambling helped. Let me know if I can answer anymore questions.

Where did you move here from?