You need to weigh your van. I assume the 6400 lb tow weight is from the owner's manual? And the tongue weight rating is that of the hitch receiver? If so, neither is an accurate gauge of what your van will do.
A conversion van's door sticker vehicle weight may or may not include all the extra stuff the upfitter added to it after it left the Chevy factory. What you will need to do is weigh the vehicle, and subtract that amount from the GVWR on the sticker--that number does not change. This will tell you for certain how much cargo you can add to the van, including passengers, gear, the weight of the hitch, and the tongue weight of the trailer.
1500 lb sound like the weight limit on your receiver, and the van may or may not (probably not) be able to carry that much. I used to tow with a conversion, and I can tell you a significant amount of your payload is already taken. You will want LT tires and a transmission cooler if you don't already have them.