Before you fill them with expanding foam, you need to break down some Styrene foam in very small chunks, break down one bead, fill the tire completely full of the Styrene, put the bead back on, then fill the tire with expanding around Styrene and it will run for years. I've seen several doing it and haven't had any break downs.
You can drill small holes around the sidewall on one side, but, very small. You want the foam to have a place to expand into. You can then shove a tire plug into each hole, cut them off smooth with the sidewall and you've cut off oxygen, dirt, water, etc. from helping break down the foam.