Im looking for a powered tadpole trike truck....I know all about the light foot trikes.
You didn't indicate that you were only interested in tadpole trikes in your original post. So, don't complain; be more specific next time you post a question. The only tadpole trike that I've seen built for cargo is Greenspeed's GTU, but I don't know if it can haul 500 pounds. You'd need to ask Greenspeed. However, you'd have to add power assist yourself, I don't think that's an option that can be added at the factory. It wouldn't be hard to do.
Speaking of Lightfoot Cycles. I was just roaming around their web site. In the used demo section they have a new golden motors hub for sale $125.00 http://www.lightfootcycles.com/demos.php Sounds like a steal if you know what you are doing.
Greenspeed are quite happy to add power at the factory.
Here are my two Greenspeed trikes. The red one I took delivery of in December 09, and came with the motor built into the rear wheel, all the controls fitted and a custom built platform for the batteries, just visible under the seat.