I haven't seen a 20R for a while, but I will give it a shot. There isn't much that can go wrong on these carbs that can make them dump too much fuel. The first thing that comes to mind is to check/replace the thermostat. If I remember right the 20R carb uses a coolant warmed choke, so if the engine doesn't warm up the choke won't open. I would pull the spark plugs and see what they look like, and do a compression check at the same time. From what you descibe it sounds like you have a vac leak somewhere though. Checking plugs will tell you if you are running rich or lean, so that is a good place to start. I don't think volume carb rebuilders do a very good job, carb rebuilding is not very difficult, but does require a certain attention to detail. That being said, usually the carb is the last thing to malfunction on these vehicles, so I would do a tune up first and go from there. Don't start throwing parts at it, that is very exspensive and rarely fixes anything. The O2 sensor should be located on the ex. manifold for all the cali's I've seen. Your truck was probably sold as a cab and chassis, which I believe were not subject to as strict emissions, so it might not have a O2 sensor on it. Contrary to what Josh says, I think 90% of carbed trucks don't have an O2 sensor. All (late model) carbed trucks do have a simple egt located in or around the cat which turns off the exhaust afterburning system to keep the cat from overheating. That is probably what you found. The exhaust pipes are normal, I think. THere is an emission computer, it is usually located on the drivers side kickpanel. I don't know if a plugged cat will cause your problems. I think your money is better spent getting a carb rebuild kit and doing it yourself. Also get a new insulator plate from the dealer, should be about $20 with new gaskets. But don't condem the carb yet. Check all your vaccuum lines and ignition system thoroughly first. Check and make sure the vac advance on the distributor works as Josh said. If you would like I might be able to get you some vac hose schematics in a few days. Good luck,
Blue82