There is a ton of room there to put a new down pipe in. You can use a truck down pipe if you have a welder. You need to cut it in the middle and take out a few inches and weld it back together. Or use a splicing clamp maybe. I tried a factory down pipe off my brother's 2003 7.3l on a van, and the dimensions are the same as far as the bends and an angles and such. The only difference is that it hung down below the frame. So it would have to be shortened.
Go and look under your van with the dog house in place and see how much space you have down there around the down pipe. It's actually quite a bit of room. You can easily go 1" bigger than stock and clear everything, I think.