Building a outside auger to push coal in my house.

True, but it fit right in the space infront of the hopper, easy transfer, and I get 1/2 at a time in my truck.

I use a gravity wagon the size of yours, only without the extension on top. Mounded up over, and to the max, I get 6 ton in it. So yes, you should be able to get at least 7 ton level across. I made a chute to fit 5 gallon pails. Works really good. You will want a roof of some sort over it so the coal doesn't freeze in the hopper. The coal will eat holes in the bottom of the gravity wagon. I covered the bottom with galvanized sheet metal. That got ate up too. So I covered it again with aluminum siding trim coil. So far, that's held up well. My gravity wagon is getting kind of rough. It was rough when I bought it, but worse now. I've been keeping my eyes open for a decent one. They are expensive around me as well. You stole yours. My next one, I will line with 3/8 or 1/2 inch plywood. Then cover the plywood with tyvek house wrap for slipperiness. Or maybe aluminum again. All that being said, we may not need coal storage if these idiots shut down the coal mines.

There is a thin plastic used on farms, in the milk house etc. Super slippery when wet, slightly slippery when dry. Line with that and you're golden. No rot.