Convert it to propane, propane engines have a few different parts for a long life. OT but the plant I worked at was all electric fork trucks when I started, then epa said too many emissions from battery charging. Whole plant changed over to propane except a few electrics. Find a shop manual, you have a weeks' worth of reading the emission systems section to see what all your truck is missing. The original systems were not entirely junk, most often just misunderstood. You can't just throw on a converter here and later on an air pump with an over rich carburetor. Modern fi did wonders improving emissions. Just try to get your truck into specs.

 

I'm guessing there's a carb/tuning issue if you smell raw fuel from the exhaust. Is it stock intake/carb or aftermarket?

Assuming the engine is healthy, start with a full cleaning, rebuild and bench setting of all the carb linkages and idle/mixture screws. If it hasn't been done in awhile, give it a tune up(plugs,wires, cap,rotor). Install a high output ignition coil. Set the base timing, then tune the carb with a vacuum gauge to get the highest possible vacuum at idle. You'll have to keep going back and forth between idle speed and mixture to get everything set right. Then check and readjust the timing and then idle speed again. Get the PVC system working, add a PVC valve and route it to a sealed air cleaner, or manifold vacuum at the base of the carb. At that point the engine should be running about as good as it can. If the exhaust is still too much for you, you can throw a couple high flow cats on it, but you need to make sure your tune is dead on or you'll burn up cats in no time if it's running too rich. You could use O2 sensors to help with tuning, but they won't be able to control anything on the engine, just monitor whatever Air/Fuel Ratio it's running at.

If you want a much cleaner running, and better performing overall, engine, going to fuel injection would be a smart move.