If it were mine...
I'm no expert by any means, but I have built my own sidecar chassis, and it rides very well. If it were mine I would go with some stiffer springs on the hack swing arm. It would raise the sidecar chassis, which looking at the first pic, it could use. Also it would have the side benefit of leaning the tug to the right which would counter act your pull to the left... That's my suggestion from one budget builder to another. I do like the refrigerator idea btw. I never thought of that, I went with a coffin but considered things such as a bath tub for my tub.