I own a 2003 chevy express 1500 hi top conversion van. It has a 5.3 liter engine and the manual says the max. trailer weight is 6600 lbs. How close to this should I go and still be safe? We are looking at trailers that weigh between 6200 and 6400 lbs.
Not a good idea. That is likely a 6600 pounds with an empty van, with no accessories installed, no passengers, no gear, a 150 pound driver and a gallon of gas... Your first step is to find out what your weight, loaded for travel (with all gear and people, and a hitch installed) is. Load up and go to a scale and find out what your total weight is, and your rear axel weight.
Once you have the loaded weight, subtract that from the GCVW rating of the van (should be in the manual) and that tells you the maximum you can tow (from a legal/safety standpoint only). I'm willing to bet this won't be much over 5000 pounds. Also, see how far the rear axel weight is from its maximum, to know how much pin weight you can tolerate. Note that being 'legal' does not mean that towing this much will be pleasant...
When looking for a trailer, consider the GVWR of the trailer, not the 'dry' weight. That weight is 'empty' (no food, no water, no clothes, no sewage and no toys), and may not include the weight of all of the options installed on the trailer.
At a guess, I'm thinking that you should be looking in the 3000 to 4000 pound range, depending on what your rear gear ratio is, and how much transmission cooling you have.