I'll try to find a thread from a couple years ago, one of our members talked about the stresses put on a hitch when using a gooseneck adapter, such that it didn't sound like a good idea. There are also some reports of warranty issues if you use a gooseneck adapter.
With what you're trying to accomplish it sounds like that type of situation, adding a lot of extra forces to the factory pin box by adding an extension and a gooseneck. You might contact Reese to see if they make one, but my gut reaction is with that weight and the stress it's probably not a good idea. But I could be wrong.
Here is a post from richheck about this:
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