GM-GMC: emissions, chevy van g20, passing emissions

Question
I own a 89 Chevy Van G20.  5.7 liter engine
Having trouble getting it to pass the Memphis,TN. emission test.  Have gotten the Hydrocarbons down from 840.0 ppm to 282.00 ppm by doing the following.
Flushed engine with K&W engine cleaner.  Changed oil and oil filter. Changed PVC Valve and air filter.  Repaired leaking vaccum line. Changed oxygen sensor.  
Do you have any other suggestions to resolve this problem.

Thanks  

Answer
If your vehicle has a catalytic converter, you probably need to replace it.

If not, or if you want to try chemical warfare, there are products sold that guarantee passing emissions tests by using them in accordance with their instructions.  I cannot recall any brand names, but auto parts stores usually carry the products.

I used to live in California and, as you might imagine, the chemicals were popular there.