Thanks everyone for the replies and links. I ended up getting the service manual for '79 where an exhaust diagram can be found showing two cats clear as day. It can't get any more official than that. I'm having to see a ref because I've changed a few things over the years (nothing major) and the truck just seems to give smog stations the willies. It's a shame there isn't some sort of special cert I could get for that truck that would allow me to forgo any smog inspections as long as I kept the mileage under 5k a year.